scons: Bump C standard to gnu11 on macOS 10.15.

Fix build error on macOS 10.15 Catalina.

src/util/u_queue.c:179:7: error: implicit declaration of function 'timespec_get' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
      timespec_get(&ts, TIME_UTC);
      ^

timespec_get needs C11 starting with macOS 10.15.

/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/time.h

   193	#if (__DARWIN_C_LEVEL >= __DARWIN_C_FULL) && \
   194	        ((defined(__STDC_VERSION__) && __STDC_VERSION__ >= 201112L) || \
   195	        (defined(__cplusplus) && __cplusplus >= 201703L))
   196	/* ISO/IEC 9899:201x 7.27.2.5 The timespec_get function */
   197	#define TIME_UTC	1	/* time elapsed since epoch */
   198	__API_AVAILABLE(macosx(10.15), ios(13.0), tvos(13.0), watchos(6.0))
   199	int timespec_get(struct timespec *ts, int base);
   200	#endif

Signed-off-by: Vinson Lee vlee@freedesktop.org