Dynamically load GTK as a module. This way:

1: If GTK isn't available, we can fall back on another LoadingWindow, and
2: ldd ./stepmania doesn't list lots of extra libraries that we only
use on startup.
This is somewhat experimental.  I don't intend to modularize much in this
way.

stepmania/src/arch/LoadingWindow/LoadingWindow_GtkModule.h

@@ -0,0 +1,8 @@
+#ifndef LOADING_WINDOW_MODULE_GTK
+#define LOADING_WINDOW_MODULE_GTK
+
+typedef const char *(*INIT)(int *argc, char ***argv);
+typedef void (*SHUTDOWN)();
+typedef void (*SETTEXT)( const char *s );
+
+#endif