[cmake] Always respect existing CMAKE_REQUIRED_FLAGS when adding additional ones.

[hintonda]

Summary

Always respect existing CMAKE_REQUIRED_FLAGS when adding
additional ones. This is important when cross compiling where
--sysroot and -target were already added.

In particular, this is needed when cross compiling from Darwin to
Linux, since --sysroot is required to find headers and libraries.

Cmake has a similar bug in check_include_file[_cxx] where
CMAKE_REQUIRED_LIBRARIES isn't passed, which causes
try_compile to fail.
(please see https://gitlab.kitware.com/cmake/cmake/merge_requests/1620)

[compnerd]

cmake/modules/CheckAtomic.cmake

11

I really wish that we could reuse the language properties instead.

cmake/config-ix.cmake

 include(CheckCXXSourceCompiles)
 include(TestBigEndian)
 
 include(CheckCompilerVersion)
 include(HandleLLVMStdlib)
 
 if( UNIX AND NOT (BEOS OR HAIKU) )
   # Used by check_symbol_exists:
-  set(CMAKE_REQUIRED_LIBRARIES m)
+  list(APPEND CMAKE_REQUIRED_LIBRARIES "m")
 endif()
 # x86_64 FreeBSD 9.2 requires libcxxrt to be specified explicitly.
 if( CMAKE_SYSTEM MATCHES "FreeBSD-9.2-RELEASE" AND
     CMAKE_SIZEOF_VOID_P EQUAL 8 )
   list(APPEND CMAKE_REQUIRED_LIBRARIES "cxxrt")
 endif()
 
 # Helper macros and functions

cmake/modules/CheckAtomic.cmake

 # atomic builtins are required for threading support.
 
 INCLUDE(CheckCXXSourceCompiles)
 INCLUDE(CheckLibraryExists)
 
 # Sometimes linking against libatomic is required for atomic ops, if
 # the platform doesn't support lock-free atomics.
 
 function(check_working_cxx_atomics varname)
   set(OLD_CMAKE_REQUIRED_FLAGS ${CMAKE_REQUIRED_FLAGS})
-  set(CMAKE_REQUIRED_FLAGS "-std=c++11")
+  set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -std=c++11")
   CHECK_CXX_SOURCE_COMPILES("
 #include <atomic>
 std::atomic<int> x;
 int main() {
   return x;
 }
 " ${varname})
   set(CMAKE_REQUIRED_FLAGS ${OLD_CMAKE_REQUIRED_FLAGS})

cmake/modules/CheckCompilerVersion.cmake

elseif(CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
         set(CLANG_CL 1)
       elseif(NOT LLVM_ENABLE_LIBCXX)
         # Otherwise, test that we aren't using too old of a version of libstdc++
         # with the Clang compiler. This is tricky as there is no real way to
         # check the version of libstdc++ directly. Instead we test for a known
         # bug in libstdc++4.6 that is fixed in libstdc++4.7.
         set(OLD_CMAKE_REQUIRED_FLAGS ${CMAKE_REQUIRED_FLAGS})
         set(OLD_CMAKE_REQUIRED_LIBRARIES ${CMAKE_REQUIRED_LIBRARIES})
-        set(CMAKE_REQUIRED_FLAGS "-std=c++0x")
+        set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -std=c++0x")
         check_cxx_source_compiles("
 #include <atomic>
 std::atomic<float> x(0.0f);
 int main() { return (float)x; }"
           LLVM_NO_OLD_LIBSTDCXX)
         if(NOT LLVM_NO_OLD_LIBSTDCXX)
           message(FATAL_ERROR "Host Clang must be able to find libstdc++4.8 or newer!")
         endif()