Added support for makepkg's PKGDEST and SRCDEST params.

This ensures that sources and packages built inside makechrootpkg go where
the user expects them to, as defined in their /etc/makepkg.conf and
~/.makepkg.conf files.

Signed-off-by: Travis Willard <travis@archlinux.org>
Signed-off-by: Aaron Griffin <aaronmgriffin@gmail.com>