Bindgen tries to use MSVC headers when using x86_64-pc-windows-gnu target

I'm using bindgen as part of a build.rs script. I would like to test that it works with x86_64-pc-windows-gnu, however it does not appear to.

My default rustup toolchain is stable-x86_64-pc-windows-gnu, however bindgen appears to be grabbing MSVC headers instead.

Example output:

D:\Code\libui-rs>cargo run --example inputs
   Compiling ui-sys v0.2.1 (D:\Code\libui-rs\ui-sys)
error: failed to run custom build command for `ui-sys v0.2.1 (D:\Code\libui-rs\ui-sys)`

Caused by:
  process didn't exit successfully: `D:\Code\libui-rs\target\debug\build\ui-sys-d4418d11d6941dbf\build-script-build` (exit code: 101)
--- stderr
C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.24.28314\include\vadefs.h:28:33: error: expected ';' after top level declarator
C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.24.28314\include\vcruntime.h:93:1: error: expected identifier
C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.24.28314\include\vcruntime.h:93:20: error: expected ';' after top level declarator
C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.24.28314\include\vcruntime.h:190:29: error: expected ';' after top level declarator
C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.24.28314\include\vcruntime.h:191:30: error: typedef redefinition with different types ('__int64' (aka 'unsigned int') vs 'long long')
C:\Program Files\LLVM\lib\clang\8.0.0\include\stddef.h:51:26: note: previous definition is here
C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.24.28314\include\vcruntime.h:339:51: error: unknown type name 'uintptr_t'
C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.24.28314\include\vcruntime.h:340:67: error: unknown type name 'uintptr_t'
C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.24.28314\include\vcruntime.h:344:8: error: unknown type name 'uintptr_t'
C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.24.28314\include\vcruntime.h:352:1: error: a parameter list without types is only allowed in a function definition
C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.24.28314\include\vcruntime.h:352:18: error: expected ';' after top level declarator
./libui/ui.h:84:2: error: type name requires a specifier or qualifier
./libui/ui.h:84:14: warning: type specifier missing, defaults to 'int' [-Wimplicit-int]
./libui/ui.h:98:12: error: unknown type name 'uintptr_t'
C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.24.28314\include\vadefs.h:28:33: error: expected ';' after top level declarator, err: true
C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.24.28314\include\vcruntime.h:93:1: error: expected identifier, err: true
C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.24.28314\include\vcruntime.h:93:20: error: expected ';' after top level declarator, err: true
C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.24.28314\include\vcruntime.h:190:29: error: expected ';' after top level declarator, err: true
C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.24.28314\include\vcruntime.h:191:30: error: typedef redefinition with different types ('__int64' (aka 'unsigned int') vs 'long long'), err: true
C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.24.28314\include\vcruntime.h:339:51: error: unknown type name 'uintptr_t', err: true
C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.24.28314\include\vcruntime.h:340:67: error: unknown type name 'uintptr_t', err: true
C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.24.28314\include\vcruntime.h:344:8: error: unknown type name 'uintptr_t', err: true
C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.24.28314\include\vcruntime.h:352:1: error: a parameter list without types is only allowed in a function definition, err: true
C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.24.28314\include\vcruntime.h:352:18: error: expected ';' after top level declarator, err: true
./libui/ui.h:84:2: error: type name requires a specifier or qualifier, err: true
./libui/ui.h:84:14: warning: type specifier missing, defaults to 'int' [-Wimplicit-int], err: false
./libui/ui.h:98:12: error: unknown type name 'uintptr_t', err: true
thread 'main' panicked at 'Unable to generate bindings: ()', ui-sys\build.rs:41:20
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace


D:\Code\libui-rs>rustup show
Default host: x86_64-pc-windows-gnu
rustup home:  C:\Users\smcro\.rustup

installed toolchains
--------------------

stable-x86_64-pc-windows-gnu (default)
stable-x86_64-pc-windows-msvc
nightly-x86_64-pc-windows-msvc

active toolchain
----------------

stable-x86_64-pc-windows-gnu (default)
rustc 1.42.0 (b8cedc004 2020-03-09)


D:\Code\libui-rs>

If I remove bindgen from my build.rs, then the cc crate is able to properly call gcc.exe and compile the rest of the crate, however I no longer have generated C bindings.

Start at ui-sys/build.rs:41 and inspect how bindgen is invoked during the build, then reproduce the failure with the x86_64-pc-windows-gnu toolchain. Check the handling of the headers shown in the error, including libui/ui.h and the MSVC vadefs.h and vcruntime.h files. Done means bindings can be generated for the GNU target without the reported MSVC-header errors.