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
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