[runtimes] Fixes building with CMake 3.26.

[Mordante]

Summary

Using -nostdinc++ as a linker flag in CMake 3.26 will cause compilation
failures due to -Wunused-command-line-argument. This causes several
compiler flags to be rejected. This will cause libunwind to refuse to
build since its required flags aren't supported by the compiler.

Note CMake 3.26 has not been released yet. This has been tested with
RC5. CMake 3.26 is used to test with C++ modules in libc++.

[ldionne]

runtimes/CMakeLists.txt

147

I think it would be really nice to instead avoid passing -nostdinc++ to the linker when we don't mean it. That would require a bit of reworking of how we detect compiler options, but that might be a good idea nonetheless. Thoughts?

[Mordante]

runtimes/CMakeLists.txt

147

I initially looked at that solution, but based on the documentation there's no real option for that
https://cmake.org/cmake/help/latest/module/CheckCSourceCompiles.html

Do you have a suggestion how to achieve that solution?

[phosek]

I have D142957 that's trying to address the same underlying issue and with that change I can build runtimes with CMake 3.26. It needs to be broken up and landed piecemeal which I started (so far I have D143052 and D144440) but haven't finished yet due to a lack of time. I can try to prioritize it.

[Mordante]

In D145596#4182043, @phosek wrote:

I have D142957 that's trying to address the same underlying issue and with that change I can build runtimes with CMake 3.26. It needs to be broken up and landed piecemeal which I started (so far I have D143052 and D144440) but haven't finished yet due to a lack of time. I can try to prioritize it.

It would be good if it can be prioritized. I like to use CMake 3.26 to work on C++ modules in libc++. For now I keep merge this patch with D144994. (Note CMake 3.26 has not been released yet and I want at least wait until it's released.)

runtimes/CMakeLists.txt

 # Disable use of the installed C++ standard library when building runtimes.
 # 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 "${CMAKE_REQUIRED_FLAGS} -nostdlib++")
 endif()
+# When present, the -nostdinc++ flag gets propagated to the linker flags. In
+# CMake 3.26 this causes try_compile to fail for other compile options. Which
+# results in flags not being set, even when present.
+check_c_compiler_flag("-Wno-unused-command-line-argument" C_SUPPORTS_START_NO_UNUSED_COMMAND_LINE_ARGUMENTS)
+if (C_SUPPORTS_START_NO_UNUSED_COMMAND_LINE_ARGUMENTS)
+  set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -Wno-unused-command-line-argument")
+endif()
 check_cxx_compiler_flag(-nostdinc++ CXX_SUPPORTS_NOSTDINCXX_FLAG)
 if (CXX_SUPPORTS_NOSTDINCXX_FLAG)
   set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nostdinc++")
 endif()
 
 # Avoid checking whether the compiler is working.
 set(LLVM_COMPILER_CHECKED ON)