add ccache support

- requires this patch to be applied to makechrootpkg:
  https://mailman.archlinux.org/pipermail/arch-projects/2013-July/003829.html
add support for pkgs using the pkgrel() function

prepare for multipkg pkgs (hopefully rare in aur)
eg: https://projects.archlinux.org/svntogit/packages.git/tree/trunk/PKGBUILD?h=packages/llvm