Change SKIP_GTK to SKIP_GTK2 in configure.
diff --git a/src/configure.in b/src/configure.in
index b99bcb8..a18c683 100644
--- a/src/configure.in
+++ b/src/configure.in
@@ -1876,7 +1876,7 @@
 fi
 
 
-if test -z "$SKIP_ATHENA" -o -z "$SKIP_NEXTAW" -o -z "$SKIP_MOTIF" -o -z "$SKIP_GTK"; then
+if test -z "$SKIP_ATHENA" -o -z "$SKIP_NEXTAW" -o -z "$SKIP_MOTIF" -o -z "$SKIP_GTK2"; then
   dnl Check for X11/xpm.h and X11/Sunkeysym.h with the GUI include path
   cppflags_save=$CPPFLAGS
   CPPFLAGS="$CPPFLAGS $X_CFLAGS"