patch 8.1.0664: configure "fail-if-missing" does not apply to enable-gui

Problem:    Configure "fail-if-missing" does not apply to the enable-gui
            argument. (Rhialto)
Solution:   Make configure fail if a GUI was specifified and "fail-if-missing"
            is enabled and the GUI test fails.
diff --git a/src/version.c b/src/version.c
index 4c5ee1b..b976ba5 100644
--- a/src/version.c
+++ b/src/version.c
@@ -800,6 +800,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    664,
+/**/
     663,
 /**/
     662,