Make distribution tarball compression format configurable

By default, bzip2 compression is selected (which is what has been used
till now). Additionally, by setting the TAR_COMP make variable to one of
the values "bzip2", "gzip", or "xz", an explicit compression format can
be requested for the distribution tarballs. For instance, invoking

  make TAR_COMP=xz sdist-ghc

will result in a tarball `./sdistprep/ghc-7.9.20140226-src.tar.xz`

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>