textures: Rewrite Freedoom's texture build system.

Stop using deutex's built-in texture builder and generate our own
texture lumps. This is essential for compatibility reasons: the
entries in the texture/pnames lumps must match the order of those
in the original IWADs. Failure to match ordering means that some
well-known WADs (eg. DTWID) would not work with Freedoom.

This fixes #1, and also means that Freedoom can now be built in
parallel using make's '-j' option.
Merge branch 'master' of github.com:freedoom/freedoom