Fixed bug 5366 - cmake build doesn't detect Metal on macOS

Tom Seddon

2nd time lucky, perhaps. patch 2 applies to current HEAD at time of writing - 4eb049c9bb1ca94efe3c40b57beda3169984d0cb from https://github.com/SDL-mirror/SDL.

This basically goes back to what was there originally, but now manually adding "-x objective-c" to the clang command line rather than "-ObjC". clang is then invoked without the "-x c" that was causing the problem, the snippet builds, and Metal is detected. (I had a quick trawl through the cmake code, but I couldn't see where this is handled.)

I was moved to try this after finding SDL's own CHECK_OBJC_SOURCE_COMPILES macro, and noting what it does: 4eb049c9bb/cmake/macros.cmake (L67)

An alternative fix of course would be to use CHECK_OBJC_SOURCE_COMPILES instead of cmake's check_objc_source_compiles - but that had the same problem of getting confused by "return 0;". (Maybe that's because it's a macro? I'll defer to a cmake expert on this one.)

I decided in the end to err on the side of leaving things looking basically the same as they were before my first patch.

CMakeLists.txt

@@ -32,7 +32,6 @@ include(CheckIncludeFiles)
 include(CheckIncludeFile)
 include(CheckSymbolExists)
 include(CheckCSourceCompiles)
-include(CheckOBJCSourceCompiles)
 include(CheckCSourceRuns)
 include(CheckCCompilerFlag)
 include(CheckTypeSize)
@@ -1802,7 +1801,9 @@ elseif(APPLE)
     endif()
 
     if(VIDEO_VULKAN OR VIDEO_METAL OR RENDER_METAL)
-      check_objc_source_compiles("
+      set(ORIG_CMAKE_REQUIRED_FLAGS ${CMAKE_REQUIRED_FLAGS})
+      set(CMAKE_REQUIRED_FLAGS "-x objective-c ${CMAKE_REQUIRED_FLAGS}")
+      check_c_source_compiles("
         #include <AvailabilityMacros.h>
         #import <Metal/Metal.h>
         #import <QuartzCore/CAMetalLayer.h>
@@ -1812,8 +1813,10 @@ elseif(APPLE)
         #endif
         int main()
         {
+            return 0;
         }
         " HAVE_FRAMEWORK_METAL)
+      set(CMAKE_REQUIRED_FLAGS ${ORIG_CMAKE_REQUIRED_FLAGS})
       if(HAVE_FRAMEWORK_METAL)
         set(SDL_FRAMEWORK_METAL 1)
         set(SDL_FRAMEWORK_QUARTZCORE 1)