Require a recent libmemcache version.
Support building on older systems.

RHEL6 has a very old libmemcached, but we don't need
full functionality there.

Conflicts:
	src/Makefile.am
	src/vmod_memcached.c
Use the same cutoff everywhere.

In the code bigger than 0.49.0 is used.

Ubuntu Precise has 0.44.0 which slipped in between these two
checks.
Log for both old and new library version.