Broken --enable-suppress-external-warning for Apple Clang 15 on x86_64

[hebasto]

Apple Clang 15 does not support -Xclang -internal-isystem anymore. On both x86_64 and arm64 platforms, the following check:

bitcoin/configure.ac

Lines 742 to 744 in 65c05db

	if test "$suppress_external_warnings" != "no"; then
	AX_CHECK_PREPROC_FLAG([-Xclang -internal-isystem/usr/local/include], [CORE_CPPFLAGS="$CORE_CPPFLAGS -Xclang -internal-isystem/usr/local/include"], [], [$CXXFLAG_WERROR])
	fi

fails with error: unknown argument: '-internal-isystem/usr/local/include'.

It makes the --enable-suppress-external-warnings option not working for dependency packages installed by Homebrew and linked to /usr/local, which happens for all non-keg packages on x86_64. There is no such an effect on arm64.

For a related discussion refer to #29165 (comment).

[fanquake]

fails with error: unknown argument: '-internal-isystem/usr/local/include'

Can this be investigated further. The -Xclang and -internal-isystem options still exist in Clang. Is the issue here just the concatenation? Maybe that worked previously, but no-longer does? What happens if you change -Xclang -internal-isystem/usr/local/include to -Xclang -internal-isystem /usr/local/include?

[hebasto]

Here are more details:

% clang -cc1 --help | grep -A 1 internal-isystem
  -internal-isystem <directory>
                          Add directory to the internal system include search path; these are assumed to not be user-provided and are used to model system and standard headers' paths.

Is the issue here just the concatenation? Maybe that worked previously, but no-longer does? What happens if you change -Xclang -internal-isystem/usr/local/include to -Xclang -internal-isystem /usr/local/include?

% clang++ -Xclang -internal-isystem /usr/local/include hello.cpp       
ld: file cannot be mmap()ed, errno=22 path=/usr/local/include in '/usr/local/include'
clang: error: linker command failed with exit code 1 (use -v to see invocation)

[hebasto]

According to Wikipedia, the Apple clang version 15.0.0 (clang-1500.0.40.1) corresponds to LLVM clang 16.0.0.

On Ubuntu 23.10:

$ clang++-16 -Xclang -internal-isystem/usr/local/include test.cpp 
error: unknown argument: '-internal-isystem/usr/local/include'  # Same for clang++-17.

However,

$ clang++-15 -Xclang -internal-isystem/usr/local/include test.cpp  # Works.

[hebasto]

Fixed in #29195.