Pacman 4.1

Necessary changes for pacman 4.1
Didn't test it yet though

What will be used as pkgver is still unclear. Some prefer the git describe thing, which fails for julia
The other grep-sed combination will print the day of the last commit

Added libm because it appeared in make.inc

removed $pkgdir from Build. It shouldn't be necessary. See https://projects.archlinux.org/svntogit/community.git/commit/trunk?h=packages/mupdf&id=8636c08353393b794be8948923db20a75a26a53e
Building is done in a fakeroot environment.

And the reminder. This only works with pacman4.1 (rc1 candidate only available at the moment http://allanmcrae.com/2013/03/pacman-4-1-0rc1/ ) so don't pull it yet :)