mesa: enable glvnd support

Allows mesa to be built with glvnd support.
Thus, creates libEGL_mesa.so* and libGLX_mesa.so*
mesa(vendor) libraries meant to coexist with vendor
neutral dispatch libraries from libglvnd.

Signed-off-by: Vincent Davis Jr <vince@underview.tech>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>