Add some temporary autoconf checks to try to figure out why test_math is

Add some temporary autoconf checks to try to figure out why test_math is
failing on Debian/alpha.  (log(9.88e-324) gives an unexpected ValueError
on that platform).  These checks will be removed again once the source
of the problem is identified.

I hope this is a reasonable way to approach the Debian/alpha buildbot
failures; if there's a better way of debugging buildbot test failures
then please let me know.