Bindgen passes the rust target to clang. It should pass the host target

I'm trying to build bindings for minimp3 (https://github.com/lieff/minimp3). I am using these bindings from an embedded project, with target "thumbv7em-none-eabihf". I call bindgen from build.rs, together with the cc invocation to compile the C library. I get a large amount of errors from clang about types being in the wrong architecture (see below). When calling bindgen with the same arguments (taken from .command_line_flags() method) the generation finishes successfully. When I force bindgen to pass the HOST env var to the clang compiler as the --target flag the generation is also successful (see build.rs). I think my solution is a hack and the default behavior of bindgen should be to pass the host target to clang.
I believe this issue is similar to issue #1555

Bindgen Invocation

bindgen --output src/bindings.rs bindgen.h --rust-target 1.33 --no-derive-default --ctypes-prefix cty --generate functions,types,vars,methods,constructors,destructors --use-core -- -Iminimp3

Build.rs usage

extern crate bindgen;
extern crate cc;
use std::env;
use std::path::PathBuf;

fn main() {
    println!("cargo:rustc-link-lib=minimp3");
    println!("cargo:rerun-if-changed=minimp3.c");
    println!("cargo:rerun-if-changed=bindgen.h");
    let mut build = cc::Build::new();
    build
        .include("minimp3")
        .define("MINIMP3_IMPLEMENTATION", None)
        .define("MINIMP3_NO_STDIO", None)
        .file("minimp3.c")
        .compile("minimp3");

    let bb = bindgen::builder()
        .header("bindgen.h")
        .ctypes_prefix("cty")
        .generate_comments(true)
        .rustfmt_bindings(true)
        .clang_arg("-Iminimp3")
        // uncomment to make it work .clang_arg(format!("--target={}", env::var("HOST").unwrap()))
        .use_core();
    eprintln!(
        "please run bindgen --output src/bindings.rs {}",
        bb.command_line_flags().join(" ")
    );

    // bindgen --output src/bindings.rs bindgen.h --rust-target 1.33 --no-derive-default --ctypes-prefix cty --generate functions,types,vars,methods,constructors,destructors --use-core -- -Iminimp3
    let bindings = bb.generate().expect("Unable to generate bindings");

    let out_path = PathBuf::from("src");
    bindings
        .write_to_file(out_path.join("bindings.rs"))
        .expect("Couldn't write bindings!");
}

Errors:1)
--- stdout
cargo:rustc-link-lib=minimp3
cargo:rerun-if-changed=minimp3.c
cargo:rerun-if-changed=bindgen.h
TARGET = Some("thumbv7em-none-eabihf")
OPT_LEVEL = Some("0")
HOST = Some("x86_64-apple-darwin")
CC_thumbv7em-none-eabihf = None
CC_thumbv7em_none_eabihf = None
TARGET_CC = None
CC = None
CROSS_COMPILE = None
CFLAGS_thumbv7em-none-eabihf = None
CFLAGS_thumbv7em_none_eabihf = None
TARGET_CFLAGS = None
CFLAGS = None
CRATE_CC_NO_DEFAULTS = None
DEBUG = Some("true")
CARGO_CFG_TARGET_FEATURE = None
running: "arm-none-eabi-gcc" "-O0" "-ffunction-sections" "-fdata-sections" "-fPIC" "-g" "-fno-omit-frame-pointer" "-mthumb" "-mfloat-abi=hard" "-march=armv7e-m" "-mfpu=fpv4-sp-d16" "-I" "minimp3" "-Wall" "-Wextra" "-DMINIMP3_IMPLEMENTATION" "-DMINIMP3_NO_STDIO" "-o" "/Users/.../src/stm32f3/example-sound/minimp3-ffi/target/thumbv7em-none-eabihf/debug/build/minimp3-ffi-f769533addf55ac0/out/minimp3.o" "-c" "minimp3.c"
exit code: 0
AR_thumbv7em-none-eabihf = None
AR_thumbv7em_none_eabihf = None
TARGET_AR = None
AR = None
running: "ar" "crs" "/Users/.../src/stm32f3/example-sound/minimp3-ffi/target/thumbv7em-none-eabihf/debug/build/minimp3-ffi-f769533addf55ac0/out/libminimp3.a" "/Users/.../src/stm32f3/example-sound/minimp3-ffi/target/thumbv7em-none-eabihf/debug/build/minimp3-ffi-f769533addf55ac0/out/minimp3.o"
exit code: 0
cargo:rustc-link-lib=static=minimp3
cargo:rustc-link-search=native=/Users/.../src/stm32f3/example-sound/minimp3-ffi/target/thumbv7em-none-eabihf/debug/build/minimp3-ffi-f769533addf55ac0/out

--- stderr
please run bindgen --output src/bindings.rs bindgen.h --rust-target 1.40 --no-derive-default --ctypes-prefix cty --generate functions,types,vars,methods,constructors,destructors --use-core -- -Iminimp3
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/sys/cdefs.h:807:2: error: Unsupported architecture
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/machine/_types.h:34:2: error: architecture not supported
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/sys/_types.h:55:9: error: unknown type name '__int64_t'
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/sys/_types.h:56:9: error: unknown type name '__int32_t'
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/sys/_types.h:57:9: error: unknown type name '__int32_t'
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/sys/_types.h:60:9: error: unknown type name '__uint32_t'
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/sys/_types.h:61:9: error: unknown type name '__uint32_t'
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/sys/_types.h:62:9: error: unknown type name '__uint64_t'
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/sys/_types.h:68:9: error: unknown type name '__darwin_natural_t'
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/sys/_types.h:70:9: error: unknown type name '__uint16_t'
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/sys/_types.h:71:9: error: unknown type name '__int64_t'
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/sys/_types.h:72:9: error: unknown type name '__int32_t'
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/sys/_types.h:73:9: error: unknown type name '__uint32_t'
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/sys/_types.h:74:9: error: unknown type name '__int32_t'
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/sys/_types.h:75:9: error: unknown type name '__uint32_t'
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/sys/_types.h:76:9: error: unknown type name '__uint32_t'
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/machine/types.h:37:2: error: architecture not supported
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/sys/_types/_intptr_t.h:32:9: error: unknown type name '__darwin_intptr_t'
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/sys/cdefs.h:807:2: error: Unsupported architecture, err: true
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/machine/_types.h:34:2: error: architecture not supported, err: true
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/sys/_types.h:55:9: error: unknown type name '__int64_t', err: true
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/sys/_types.h:56:9: error: unknown type name '__int32_t', err: true
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/sys/_types.h:57:9: error: unknown type name '__int32_t', err: true
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/sys/_types.h:60:9: error: unknown type name '__uint32_t', err: true
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/sys/_types.h:61:9: error: unknown type name '__uint32_t', err: true
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/sys/_types.h:62:9: error: unknown type name '__uint64_t', err: true
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/sys/_types.h:68:9: error: unknown type name '__darwin_natural_t', err: true
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/sys/_types.h:70:9: error: unknown type name '__uint16_t', err: true
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/sys/_types.h:71:9: error: unknown type name '__int64_t', err: true
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/sys/_types.h:72:9: error: unknown type name '__int32_t', err: true
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/sys/_types.h:73:9: error: unknown type name '__uint32_t', err: true
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/sys/_types.h:74:9: error: unknown type name '__int32_t', err: true
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/sys/_types.h:75:9: error: unknown type name '__uint32_t', err: true
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/sys/_types.h:76:9: error: unknown type name '__uint32_t', err: true
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/machine/types.h:37:2: error: architecture not supported, err: true
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/sys/_types/_intptr_t.h:32:9: error: unknown type name '__darwin_intptr_t', err: true
thread 'main' panicked at 'Unable to generate bindings: ()', src/libcore/result.rs:1165:5
note: run with RUST_BACKTRACE=1 environment variable to display a backtrace.

Reproduce the cross-compilation failure from the shown build.rs, comparing bindgen’s builder().generate() behavior with command_line_flags() and an explicit HOST target. Trace how target arguments reach clang; done means the host target is selected by default without the manual clang_arg workaround, while the thumbv7em-none-eabihf target remains available for the C build.