Update PKGBUILD

This PKGBUILD is updates for the new VCS syntax for makepkg 4.1, and includes dynamic checking for config/icons location and usage. It shaves about 15 lines off the current PKGBUILD. As mentioned in #19, this will require a user config in ``$HOME`` to be named ``.ttwm_config.h``.
Merge pull request #20 from HalosGhost/patch-2

Update PKGBUILD