[runtimes] Append -nostd*++ flags only for Clang

Append `-nostdlib++` and `-nostdinc++` flags to `CMAKE_REQUIRED_FLAGS`
only if we are actually building with Clang.  These flags are also
passed to the C compiler, which is not allowed in GCC.  Since CMake
implicitly performs some tests using the C compiler, this can lead
to incorrect check results.  This should be safe, since FWIU we only
need them when bootstrapping Clang.

Even though we know that Clang supports these flags, we still need
to explicitly check if they work, as in some scenarios adding
`-nostdlib++` actually breaks the build.  See PR #108357 for examples
of that.

Fixes #90332

Signed-off-by: Michał Górny <[email protected]>

Author: mgorny

runtimes/CMakeLists.txt

@@ -160,13 +160,26 @@ endif()
 # Check for -nostdlib++ first; if there's no C++ standard library yet,
 # all check_cxx_compiler_flag commands will fail until we add -nostdlib++
 # (or -nodefaultlibs).
-llvm_check_compiler_linker_flag(CXX "-nostdlib++" CXX_SUPPORTS_NOSTDLIBXX_FLAG)
-if (CXX_SUPPORTS_NOSTDLIBXX_FLAG)
-  set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nostdlib++")
-endif()
-check_cxx_compiler_flag(-nostdinc++ CXX_SUPPORTS_NOSTDINCXX_FLAG)
-if (CXX_SUPPORTS_NOSTDINCXX_FLAG)
-  set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nostdinc++")
+#
+# CMAKE_REQUIRED_FLAGS is used both for C and C++ compilation. This breaks
+# compiling with GCC (and we should not need to force -nostd* for GCC anyway),
+# so apply it only with Clang. However, since there are cases when appending
+# the flag actually breaks the build, still perform the checks rather than
+# appending it unconditionally.
+#
+# TODO: find a better solution. See the discussion on:
+# https://github.com/llvm/llvm-project/issues/90332
+# https://github.com/llvm/llvm-project/pull/108357
+
+if ("${CMAKE_CXX_COMPILER_ID}" MATCHES "Clang")
+  llvm_check_compiler_linker_flag(CXX "-nostdlib++" CXX_SUPPORTS_NOSTDLIBXX_FLAG)
+  if (CXX_SUPPORTS_NOSTDLIBXX_FLAG)
+    set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nostdlib++")
+  endif()
+  check_cxx_compiler_flag(-nostdinc++ CXX_SUPPORTS_NOSTDINCXX_FLAG)
+  if (CXX_SUPPORTS_NOSTDINCXX_FLAG)
+    set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nostdinc++")
+  endif()
 endif()
 
 # Avoid checking whether the compiler is working.