Remove DirectFBGL header from Mesa bacause since 1.0.0 DirectFB installs its own header.

Updated to the current DirectFBGL interface (i.e. added GetProcAddress()).
diff --git a/docs/README.directfb b/docs/README.directfb
index 169ebe4..e3bada4 100644
--- a/docs/README.directfb
+++ b/docs/README.directfb
@@ -6,7 +6,7 @@
 ============
 
   To build Mesa with DirectFB (DirectFBGL) support you need:
-     - DirectFB at least 0.9.21 (http://directfb.org)
+     - DirectFB at least 1.0.0 (http://directfb.org)
      - pkg-config at least 0.9 (http://pkgconfig.sf.net)