Individual sdist-foo targets

so that one can build a source distribution without downloading the
windows tarballs.

(cherry picked from commit 50e4d40f88c3eee41384efacfd83f73305e2210d)
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>

(cherry picked from commit 32f41c79960ffc1d04c4573acb37756109d279a5)
Follow-up to 32f41c79

These parts were forgotten to be committed together with the rest of
32f41c79960ffc1d04c4573acb37756109d279a5

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

(cherry picked from commit b1ee32edbe9be7b695cce7975450782e0a290c47)