Check for O_CLOEXEC definition in fcntl.h at configuration time,

If not available, define it internally as 0 (in SDL_internal.h).
Define it as available for macOS >= 10.7 in SDL_config_macosx.h.

include/SDL_config_macosx.h

@@ -144,6 +144,10 @@
 # endif
 #endif
 
+#if (MAC_OS_X_VERSION_MAX_ALLOWED >= 1070)
+#define HAVE_O_CLOEXEC 1
+#endif
+
 #define HAVE_GCC_ATOMICS 1
 
 /* Enable various audio drivers */