patch 9.0.1450: MacOS: building fails if clock_gettime() is not available

Problem:    MacOS: building fails if clock_gettime() is not available.
Solution:   Add a configure check for clock_gettime(). (closes #12242)
diff --git a/src/configure.ac b/src/configure.ac
index 0c776a2..6936ac9 100644
--- a/src/configure.ac
+++ b/src/configure.ac
@@ -3709,7 +3709,8 @@
 	getpgid setpgid setsid sigaltstack sigstack sigset sigsetjmp sigaction \
 	sigprocmask sigvec strcasecmp strcoll strerror strftime stricmp strncasecmp \
 	strnicmp strpbrk strptime strtol tgetent towlower towupper iswupper \
-	tzset usleep utime utimes mblen ftruncate unsetenv posix_openpt)
+	tzset usleep utime utimes mblen ftruncate unsetenv posix_openpt \
+	clock_gettime)
 AC_FUNC_SELECT_ARGTYPES
 AC_FUNC_FSEEKO