patch 8.2.2337: configure test for GTK only says "no"

Problem:    Configure test for GTK only says "no". (Harm te Hennepe)
Solution:   Hint that a -def package is needed. (closes #5229)
diff --git a/src/configure.ac b/src/configure.ac
index d13e378..fe2a401 100644
--- a/src/configure.ac
+++ b/src/configure.ac
@@ -2583,6 +2583,9 @@
 	     sed 's/\([[0-9]]*\)\.\([[0-9]]*\)\.\([[0-9]]*\)/\3/'`
     }
     else
+      dnl Put some text before the "no" to hint at installing the gtk-dev
+      dnl packages.
+      AC_MSG_CHECKING(for GTK -dev package)
       no_gtk=yes
     fi