Break up gtkmisc.c.

It's another file that should have been subdivided into lots of tiny
separate things in the utils library - especially since for some
reason I made a completely separate 'guimisc' cmake-level library for
it when there was no need.

unix/utils/buildinfo_gtk_version.c

@@ -0,0 +1,14 @@
+/*
+ * Return the version of GTK we were compiled against, for buildinfo.
+ */
+
+#include <gtk/gtk.h>
+#include "putty.h"
+#include "gtkcompat.h"
+#include "gtkmisc.h"
+
+char *buildinfo_gtk_version(void)
+{
+    return dupprintf("%d.%d.%d",
+                     GTK_MAJOR_VERSION, GTK_MINOR_VERSION, GTK_MICRO_VERSION);
+}