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/gtkmisc.c

@@ -200,3 +200,9 @@ void our_dialog_add_to_content_area(GtkWindow *dlg, GtkWidget *w,
          w, expand, fill, padding);
 #endif
 }
+
+char *buildinfo_gtk_version(void)
+{
+    return dupprintf("%d.%d.%d",
+                     GTK_MAJOR_VERSION, GTK_MINOR_VERSION, GTK_MICRO_VERSION);
+}

unix/unix.h

@@ -32,17 +32,22 @@
 #define DEFAULT_CLIPBOARD GDK_SELECTION_CLIPBOARD /* OS X has no PRIMARY */
 
 #define BUILDINFO_PLATFORM "OS X (GTK)"
+#define BUILDINFO_GTK
 
 #elif defined NOT_X_WINDOWS
 
 #define BUILDINFO_PLATFORM "Unix (pure GTK)"
+#define BUILDINFO_GTK
 
 #else
 
 #define BUILDINFO_PLATFORM "Unix (GTK + X11)"
+#define BUILDINFO_GTK
 
 #endif
 
+char *buildinfo_gtk_version(void);
+
 struct Filename {
     char *path;
 };

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;
+}