[libcxx] [libcxxabi] [libunwind] [llvm] [runtimes] Probe for -nostdlib++ and -nostdinc++ with the C compiler (PR #108357)

[Louis Dionne via cfe-commits]

================
@@ -38,9 +38,13 @@ check_cxx_compiler_flag(-nolibc CXX_SUPPORTS_NOLIBC_FLAG)
 # required during compilation (which has the -nostdlib++ or -nodefaultlibs). libc is
 # required for the link to go through. We remove sanitizers from the
 # configuration checks to avoid spurious link errors.
+#
+# Adding flags to CMAKE_REQUIRED_FLAGS will include the flags both when testing
+# compilation of C and C++. Therefore test to make sure that the flags are
+# supported by the C compiler driver, before deciding to include them.
 
-check_cxx_compiler_flag(-nostdlib++ CXX_SUPPORTS_NOSTDLIBXX_FLAG)
-if (CXX_SUPPORTS_NOSTDLIBXX_FLAG)
+check_c_compiler_flag(-nostdlib++ C_SUPPORTS_NOSTDLIBXX_FLAG)
----------------
ldionne wrote:

> The problem is that even if we don’t use any C sources outright, cmake uses C compilation tests when you do a check for whether a library exists.

Do you mean these checks? https://github.com/llvm/llvm-project/blob/aa02b76b1a4c2df1a701fbd8a8378d1cd946e70a/libcxx/cmake/config-ix.cmake#L130

https://github.com/llvm/llvm-project/pull/108357