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.h.cmake

@@ -222,6 +222,7 @@
 #cmakedefine HAVE_INOTIFY_INIT 1
 #cmakedefine HAVE_INOTIFY_INIT1 1
 #cmakedefine HAVE_INOTIFY 1
+#cmakedefine HAVE_O_CLOEXEC 1
 
 /* Apple platforms might be building universal binaries, where Intel builds
    can use immintrin.h but other architectures can't. */