clean up session-key configure checks

[Daniel Kahn Gillmor]

The Ubuntu linker prefers --as-needed.  This is nice, but it means
that our _check_session_key.c configure-time script (introduced by me
in 30c4fa3702f90572afcd1984dbd7aba70f2b4fd9) fails on xenial.

In the course of debugging the reason, i found several other
infelicities in the error cases in handling that bit of ./configure,
so i'm cleaning those up with this series too.

   --dkg