Fix -fuse-ld feature detection error.

[ruiu]

Summary

check_cxx_compiler_flag doesn't seem to try to link a program, so
the existing code doesn't correctly detect the availability of a given
linker. This patch uses check_cxx_source_compiles instead.

I confirmed that cmake now reports this error

Host compiler does not support '-fuse-ld=foo'

for -DLLVM_USE_LINKER=foo.

[lichray]

Try this advice https://cmake.org/pipermail/cmake/2011-July/045525.html?

[ruiu]

Are you suggesting I use check_cxx_comipler_flags instead of check_cxx_source_compiles, or something different?

[lichray]

In D39274#906587, @ruiu wrote:

Are you suggesting I use check_cxx_comipler_flags instead of check_cxx_source_compiles, or something different?

Never mind, I didn't saw you already be using CMAKE_REQUIRED_FLAGS.

llvm/trunk/cmake/modules/HandleLLVMOptions.cmake

 if( LLVM_ENABLE_LLD )
   if ( LLVM_USE_LINKER )
     message(FATAL_ERROR "LLVM_ENABLE_LLD and LLVM_USE_LINKER can't be set at the same time")
   endif()
   set(LLVM_USE_LINKER "lld")
 endif()
 
 if( LLVM_USE_LINKER )
-  check_cxx_compiler_flag("-fuse-ld=${LLVM_USE_LINKER}" CXX_SUPPORTS_CUSTOM_LINKER)
+  set(OLD_CMAKE_REQUIRED_FLAGS ${CMAKE_REQUIRED_FLAGS})
+  set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -fuse-ld=${LLVM_USE_LINKER}")
+  check_cxx_source_compiles("int main() { return 0; }" CXX_SUPPORTS_CUSTOM_LINKER)
   if ( NOT CXX_SUPPORTS_CUSTOM_LINKER )
     message(FATAL_ERROR "Host compiler does not support '-fuse-ld=${LLVM_USE_LINKER}'")
   endif()
+  set(CMAKE_REQUIRED_FLAGS ${OLD_CMAKE_REQUIRED_FLAGS})
   append("-fuse-ld=${LLVM_USE_LINKER}"
     CMAKE_EXE_LINKER_FLAGS CMAKE_MODULE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS)
 endif()
 
 if( LLVM_ENABLE_PIC )
   if( XCODE )
     # Xcode has -mdynamic-no-pic on by default, which overrides -fPIC. I don't
     # know how to disable this, so just force ENABLE_PIC off for now.