Include the compile-time GTK version in the build info.

It's obvious to the trained eye whether GTK PuTTY was compiled against
GTK2 or GTK3, but the untrained eye would probably appreciate a little
help, and even the trained eye probably can't tell GTK 3.18 from 3.19
at a glance :-)

unix/uxnogtk.c

@@ -0,0 +1,11 @@
+/*
+ * uxnogtk.c: link into non-GUI Unix programs so that they can tell
+ * buildinfo about a lack of GTK.
+ */
+
+#include "putty.h"
+
+char *buildinfo_gtk_version(void)
+{
+    return NULL;
+}