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/config.h.in b/src/config.h.in
index 6cb43da..d0257cc 100644
--- a/src/config.h.in
+++ b/src/config.h.in
@@ -225,6 +225,7 @@
 #undef HAVE_BIND_TEXTDOMAIN_CODESET
 #undef HAVE_MBLEN
 #undef HAVE_TIMER_CREATE
+#undef HAVE_CLOCK_GETTIME
 
 /* Define, if needed, for accessing large files. */
 #undef _LARGE_FILES