Replace MASTERMOD with a per-game constant

Yup, packfiles finally proved that we really have a different set of changes
to master.lib in every game. Also, there are bound to be more of these game-
specific small changes to otherwise identical code in ZUN's own code.

And hey, no need to define that value in the build scripts anymore.

(I've also considered just copying modified versions into the individual game
subdirectories, but it's not too nice to expect people to diff them in order
to actually understand why these copies exist and where the changes actually
are.)