cmake: do all compile tests with -D_GNU_SOURCE=1

CMakeLists.txt

@@ -131,16 +131,17 @@ else()
   set(PROCESSOR_ARCH "x86")
 endif()
 
-# Get the platform
-SDL_DetectCMakePlatform()
-
 set(SDL_CHECK_REQUIRED_INCLUDES "" CACHE STRING "Extra includes (for CMAKE_REQUIRED_INCLUDES)")
 set(SDL_CHECK_REQUIRED_LINK_OPTIONS "" CACHE STRING "Extra link options (for CMAKE_REQUIRED_LINK_OPTIONS)")
 mark_as_advanced(SDL_CHECK_REQUIRED_INCLUDES SDL_CHECK_REQUIRED_LINK_OPTIONS)
 
+set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -D_GNU_SOURCE=1")
 list(APPEND CMAKE_REQUIRED_INCLUDES ${SDL_CHECK_REQUIRED_INCLUDES})
 list(APPEND CMAKE_REQUIRED_LINK_OPTIONS ${SDL_CHECK_REQUIRED_LINK_OPTIONS})
 
+# Get the platform
+SDL_DetectCMakePlatform()
+
 # Don't mistake macOS for unix
 if(UNIX AND NOT ANDROID AND NOT APPLE AND NOT RISCOS)
   set(UNIX_SYS ON)