move mesa10 build into .old scheme:

- add ${X11SRCDIR.MesaLib.old} and introduce ${X11SRCDIR.Mesa} as an
alias for either the former or the non old. this allows many of
the makefiles to simply use ${X11SRCDIR.Mesa} (but does not really
enable much sharing of makefiles, but reduces their diffs.)

- use mesa-which.mk to define ${OLD_PREFIX} to either "" or ".old",
and to know if to build 'dri7' (.old only.) ${OLD_PREFIX} is used
by other code (eg, LIBDPLIBS) to pick the right subdir.

branches: 1.1.1;
Initial revision

import the xorg-server 1.10 reach over tree as xorg-server.old.