r10377: Save configuration stuff to sconf.cache so it isn't annoyingly run

r10377: Save configuration stuff to sconf.cache so it isn't annoyingly run
at every single build.  Run 'scons configure=1' or delete sconf.cache
to force checks to be re-run.

Jelmer, I think this stuff is cached in the .sconf_cache directory but
the message is still displayed and it looks like it caches the compiled
test object file not the actual result of the test.
(This used to be commit 9d001dc083937bbf5642af90bc8a8b1a27825de0)