Retain linker flags specified on cmake command line.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@183857 91177308-0d34-0410-b5e6-96231b3b80d8

CMakeLists.txt

@@ -151,7 +151,7 @@ if (LLVM_COMPILER_IS_GCC_COMPATIBLE)
 endif ()
 
 if (APPLE)
-  set(CMAKE_MODULE_LINKER_FLAGS "-Wl,-flat_namespace -Wl,-undefined -Wl,suppress")
+  set(CMAKE_MODULE_LINKER_FLAGS "${CMAKE_MODULE_LINKER_FLAGS} -Wl,-flat_namespace -Wl,-undefined -Wl,suppress")
 endif ()
 
 configure_file(