Update PKGBUILD

This PKGBUILD is updated 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``.

The final check to return to the user that no customizations were found is rather long and gross, but I could not think of a better way of doing it.