Specify the Snappy libdir install location

Addresses: basho/eleveldb#64

It was reported that on OpenSUSE snappy's configure installs to
the wrong location despite the default setting in snappy's
configure.  This change merely forces the issue to the
expected location.
Merge pull request #70 from basho/jem-explicit-snappy-libdir

Specify the Snappy libdir install location