patch 9.0.1517: MacOS: configure removes -O2 from $CFLAGS

Problem:    MacOS: configure removes -O2 from $CFLAGS.
Solution:   Only adjust $CFLAGS for gcc. (closes #12351)
diff --git a/src/version.c b/src/version.c
index 260afa8..dc17976 100644
--- a/src/version.c
+++ b/src/version.c
@@ -696,6 +696,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1517,
+/**/
     1516,
 /**/
     1515,