configure: make _check_session_keys work with an as-needed linker
When using a promiscuous linker, _check_session_keys was working fine.
But some OSes (including some versions of Ubuntu) have set their
linker to always link in "--as-needed" mode, which means that the
order of the objects linked is relevant. If a library is loaded
before it is needed, that library will no longer be linked in the
final outcome. _check_session_keys.c was failing on those systems.
Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>