mesa: Move compile of common Mesa core files to a static lib.
We were compiling them twice, costing extra build time. Reduces my ccache-hot clean build time by a second (24.3s to 23.3s, 3 runs each).
The windows args are a little strange -- it's not clear to me that they're actually used for building these files, but keep them in place just in case, since we don't have a good windows CI story yet. We should want them on both gallium and classic regardless: Only osmesa could be built for windows in classic, and classic OSMesa's scons build defines these flags too.
Closes: #2052 (closed)