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.

src/SDL_internal.h

@@ -51,6 +51,10 @@
 
 #include "SDL_config.h"
 
+#ifndef HAVE_O_CLOEXEC
+#define O_CLOEXEC                       0
+#endif
+
 /* A few #defines to reduce SDL2 footprint.
    Only effective when library is statically linked.
    You have to manually edit this file. */