autoconf: Better client API selection.

autoconf: Better client API selection.

Make autoconf decide the client APIs enabled first.  Then when OpenGL
and OpenGL ES are disabled, there is no need to build src/mesa/;  when
OpenGL is disabled, no $mesa_driver should be built.  Finally, add
--enable-openvg to enable OpenVG.

With these changes, an OpenVG only build can be configured with

  $ ./configure --disable-opengl --enable-openvg

src/mesa, src/glsl, and src/glx will be skipped, which saves a great
deal of compilation time.

And an OpenGL ES only build can be configured with

  $ ./configure --disable-opengl --enable-gles-overlay