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>
diff --git a/configure.ac b/configure.ac
index 7a89591..40e052c 100644
--- a/configure.ac
+++ b/configure.ac
@@ -2066,6 +2066,7 @@
 		src/gallium/targets/gbm/Makefile
 		src/gallium/targets/opencl/Makefile
 		src/gallium/targets/osmesa/Makefile
+		src/gallium/targets/osmesa/osmesa.pc
 		src/gallium/targets/pipe-loader/Makefile
 		src/gallium/targets/libgl-xlib/Makefile
 		src/gallium/targets/vdpau-nouveau/Makefile