Update PKGBUILD

Hi,
as your PKGBUILD in AUR didn't build here, i updated it. I used the chance to introduce some more changes:

- added libcmark. I'm not sure, perhaps this could go in an extra package. On the other side, it's just ~500KB.
- reflected that change in the description
- cmark and re2c are build dependencies
- removed check(), as it was useless