gallium/osmesa: Make and install an osmesa.pc.

As of "2f142d59 build: Add --enable-gallium-osmesa flag." the pkgconfig
file from classic osmesa is no longer installed when building gallium
osmesa, so copy it to gallium osmesa and install the copy instead.

CC: "9.2" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>
3 files changed