Stupid me. I don't know why I moved the GLView.h here at first, but now I know why

I shouldn't:
Official BeOS GLView.h defines a virtual ErrorCallback(GLenum) method, assuming
a unsigned long GLenum.
But under Mesa, GLenum is unsigned int.
To keep binary compatibility, BeOS apps including Mesa GL/gl.h MUST include *our*
GLView.h, or linker will report a missing ErrorCallback(unsigned int) symbol. Glup.

src/mesa/drivers/beos/GLView.cpp

@@ -59,7 +59,7 @@ extern "C" {
 }	// extern "C"
 
 #include <interface/Screen.h>
-#include "GLView.h"
+#include <GLView.h>
 
 // BeOS component ordering for B_RGBA32 bitmap format
 #if B_HOST_IS_LENDIAN