configure: make _check_session_keys work with an as-needed linker

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>