better check for clock_gettime_nsec_np() -- cf. bug #5467.

include/SDL_config_iphoneos.h

@@ -53,6 +53,7 @@
 #define HAVE_REALLOC    1
 #define HAVE_FREE   1
 #define HAVE_ALLOCA 1
+#define HAVE_CLOCK_GETTIME_NSEC_NP 1
 #define HAVE_GETENV 1
 #define HAVE_SETENV 1
 #define HAVE_PUTENV 1

include/SDL_config_macosx.h

@@ -56,6 +56,9 @@
 #define HAVE_CALLOC 1
 #define HAVE_REALLOC    1
 #define HAVE_FREE   1
+#if MAC_OS_X_VERSION_MIN_REQUIRED >= 101200
+#define HAVE_CLOCK_GETTIME_NSEC_NP 1
+#endif
 #define HAVE_ALLOCA 1
 #define HAVE_GETENV 1
 #define HAVE_SETENV 1