Log Message:
Allow mmix to be built when teTeX is already installed (and not tetex1)
As mmix needs cweb, let mmix use tex.buildlink.mk (teTeX2 and teTeX3 do provide
cweb), and add a dependency over cweb in case TEX_DEFAULT is tetex1
The tetex build dependency in this case is not a problem, since tetex is needed \ 
by cweb.
Also, no PKGREVISION bump, since there are only build depends
This fixes PR 3254