pgmemcache: support building with omcache instead of libmemcached

Support for a new memcache client library, OMcache, in addition to
libmemcached which remains the default.  To build with OMcache install the
OMcache development packages and build with 'make USE_OMCACHE=1'.

OMcache build does not yet support all features, notably memcache_mget()
won't be available in OMcache builds.

See https://github.com/saaros/omcache for more information about OMcache.