add a configure check for StatFS

This way, the build log will indicate whether StatFS can be relied on.
I've tested all the failing architectures now, and on all of them,
the StatFS code now returns Nothing, rather than Just nonsense.

Also, if annex.diskreserve is set on a platform where StatFS is not
working, git-annex will complain.

Also, the Makefile was missing the sources target used when building with
cabal.

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