MacOS: fails to compile

• MacOS Sequoia 15.5, Xcode-16.4.
• rustc 1.88.0 (6b00bc388 2025-06-23)

Clang:

clang version 20.1.7
Target: x86_64-apple-darwin24.5.0
Thread model: posix
InstalledDir: /opt/local/libexec/llvm-20/bin
$ sudo port select clang apple-clang
Selecting 'apple-clang' for 'clang' succeeded. 'apple-clang' is now active.
$ clang --version
Apple clang version 17.0.0 (clang-1700.0.13.5)
Target: x86_64-apple-darwin24.5.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin
$ 

Problem: C compiler hangs up trying to compile detect_compiler_family.c:

$ ifrun clang
1638356520  8311  8308   0 11:08AM ttys010    0:00.02 /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang -E /var/folders/c6/lnc_0m093ys8w16md_fm1mnxhtfnj8/T/cargo-installom9ShE/release/build/libz-sys-d98ab235bd715166/out/16425769093887032327detect_compiler_family.c
1638356520  8312  8311   0 11:08AM ttys010    0:00.01 /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang -cc1 -triple x86_64-apple-macosx15.0.0 -Wundef-prefix=TARGET_OS_ -Wdeprecated-objc-isa-usage -Werror=deprecated-objc-isa-usage -Werror=implicit-function-declaration -E -disable-free -clear-ast-before-backend -disable-llvm-verifier -discard-value-names -main-file-name 16425769093887032327detect_compiler_family.c -mrelocation-model pic -pic-level 2 -mframe-pointer=all -fno-strict-return -ffp-contract=on -fno-rounding-math -funwind-tables=2 -target-sdk-version=15.5 -fvisibility-inlines-hidden-static-local-var -fdefine-target-os-macros -fno-assume-unique-vtables -fno-modulemap-allow-subdirectory-search -target-cpu penryn -tune-cpu generic -debugger-tuning=lldb -fdebug-compilation-dir=/Users/ur20980/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/libz-sys-1.1.22 -target-linker-version 1167.5 -fcoverage-compilation-dir=/Users/ur20980/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/libz-sys-1.1.22 -resource-dir /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/clang/17 -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk -internal-isystem /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/local/include -internal-isystem /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/clang/17/include -internal-externc-isystem /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include -internal-externc-isystem /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include -Wno-reorder-init-list -Wno-implicit-int-float-conversion -Wno-c99-designator -Wno-final-dtor-non-final-class -Wno-extra-semi-stmt -Wno-misleading-indentation -Wno-quoted-include-in-framework-header -Wno-implicit-fallthrough -Wno-enum-enum-conversion -Wno-enum-float-conversion -Wno-elaborated-enum-base -Wno-reserved-identifier -Wno-gnu-folding-constant -ferror-limit 19 -stack-protector 1 -fstack-check -mdarwin-stkchk-strong-link -fblocks -fencode-extended-block-signature -fregister-global-dtors-with-atexit -fgnuc-version=4.2.1 -fskip-odr-check-in-gmf -fmax-type-align=16 -fcommon -clang-vendor-feature=+disableNonDependentMemberExprInCurrentInstantiation -fno-odr-hash-protocols -clang-vendor-feature=+enableAggressiveVLAFolding -clang-vendor-feature=+revert09abecef7bbf -clang-vendor-feature=+thisNoAlignAttr -clang-vendor-feature=+thisNoNullAttr -clang-vendor-feature=+disableAtImportPrivateFrameworkInImplementationError -D__GCC_HAVE_DWARF2_CFI_ASM=1 -o - -x c /var/folders/c6/lnc_0m093ys8w16md_fm1mnxhtfnj8/T/cargo-installom9ShE/release/build/libz-sys-d98ab235bd715166/out/16425769093887032327detect_compiler_family.c
$

The above picture does not change after 10 minutes, at which time I kill it manually.

Same with a different compiler:

$ ifrun clang
1638356520  9358  9355   0 11:24AM ttys010    0:00.76 /opt/local/libexec/llvm-20/bin/clang -E /var/folders/c6/lnc_0m093ys8w16md_fm1mnxhtfnj8/T/cargo-installs4pgCn/release/build/libz-sys-d98ab235bd715166/out/14215467920557312181detect_compiler_family.c
$

The only way to successfully pass this hurdle is to set default compiler to GCC. Which is not my preference/

Start by reproducing the hang while building libz-sys with Apple Clang or LLVM Clang, focusing on the generated detect_compiler_family.c shown in the build output. Compare the compiler invocation with the GCC case and determine what change lets the crate compile without switching the default compiler; done means the macOS build completes successfully with Clang.