Restore compatibility with older autoconfs.

The one in Ubuntu 10.04 doesn't know what AM_PROG_AR means, so
configure.ac was broken in r10053 when fixing compatibility with later
versions; you can't win...


git-svn-id: svn://svn.tartarus.org/sgt/putty@10086 cda61777-01e9-0310-a592-d414129be87e
Gracefully degrade in the absence of CryptProtectMemory.

XP doesn't have it, and I think having connection sharing work without
its privacy enhancement is better than having it not work at all.


git-svn-id: svn://svn.tartarus.org/sgt/putty@10087 cda61777-01e9-0310-a592-d414129be87e