On 10.5, don't set GTK+ etc. versions if we haven't already set them.

When building for 10.5, if the version numbers of GTK+ and its support
libraries haven't been set, don't override the settings to get older
versions; that way, if you've changed macosx-setup.sh not to bother
installing GTK+, this won't force it to be installed.

Change-Id: Ia05f45fb6bd469b568a28a8769f3130a99180d7e
Reviewed-on: https://code.wireshark.org/review/10945
Reviewed-by: Guy Harris <guy@alum.mit.edu>

macosx-setup.sh

@@ -2232,10 +2232,22 @@ then
         # ages ago, but don't remember what I did.
         #
         GLIB_VERSION=2.16.3
-        CAIRO_VERSION=1.6.4
-        ATK_VERSION=1.24.0
-        PANGO_VERSION=1.20.2
-        GTK_VERSION=2.12.9
+        if [ "$CAIRO_VERSION" ]
+        then
+            CAIRO_VERSION=1.6.4
+        fi
+        if [ "$ATK_VERSION" ]
+        then
+            ATK_VERSION=1.24.0
+        fi
+        if [ "$PANGO_VERSION" ]
+        then
+            PANGO_VERSION=1.20.2
+        fi
+        if [ "$GTK_VERSION" ]
+        then
+            GTK_VERSION=2.12.9
+        fi
 
         #
         # That version of GTK+ includes gdk-pixbuf.