dricore: Turn it into a normal library.
Our intention is still that it's not abi stable, so make the package
version number get included in the library name. Now you can parallel
install dricore-using drivers from multiple mesa versions. We can put
it into lib now that we're following library versioning rules
(assuming that ABIs don't change within a single Mesa point release).
LD_LIBRARY_PATH still doesn't work with a non-/, non-/usr prefix
because libtool uses rpath instead of runpath for nonstandard
prefixes.