Ensure gtkutil.h derives HAVE_GTK3 from GTK_MAJOR_VERSION when missing so GTK3/GTK4 builds select the correct typedefs and declarations in Windows builds that include gtkutil.h directly.

Link libX11 on non-Windows builds regardless of GTK2/GTK3 to satisfy the X11 symbols used in gtkutil.c, fixing the GTK2 linker failure.

src/fe-gtk/gtkutil.h

@@ -26,6 +26,14 @@
 
 typedef void (*filereqcallback) (void *, char *file);
 
+#ifndef HAVE_GTK3
+#if GTK_MAJOR_VERSION >= 3
+#define HAVE_GTK3 1
+#else
+#define HAVE_GTK3 0
+#endif
+#endif
+
 #if HAVE_GTK3
 typedef enum
 {