Fixed VGL detection on FreeBSD (thanks David!)

--HG--
extra : convert_revision : svn%3Ac70aab31-4412-0410-b14c-859654838e24/trunk%40145

configure.in

@@ -775,7 +775,7 @@ CheckVGL()
          #include <vgl.h>
         ],[
          VGLBitmap bitmap;
-         exit(bitmap.PixelBytes);
+         exit(bitmap.Bitmap);
         ],[
         video_vgl=yes
         ])