Fix clock_gettime presence detect on FreeBSD.

For non-Linux, _POSIX_C_SOURCE and friends restrict symbols defined rather
than enabling defines of symbols.  Additionally, CLOCK_MONOTONIC was
apparently added to the standard around 2000 anyway, not 1993.

configure.ac

@@ -722,7 +722,9 @@ if ! test "x$have_clock_gettime" = xno; then
     LIBS="$CLOCK_LIBS"
 
     AC_RUN_IFELSE([
+#ifdef __linux__
 #define _POSIX_C_SOURCE 199309L
+#endif
 #include <time.h>
 
 int main(int argc, char *argv[[]]) {