Fix -fuse-ld feature detection error.

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.

Differential Revision: https://reviews.llvm.org/D39274

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@316958 91177308-0d34-0410-b5e6-96231b3b80d8

cmake/modules/HandleLLVMOptions.cmake

@@ -194,10 +194,13 @@ if( LLVM_ENABLE_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()