GreenstackUnitTests: Don't recompile gtest
GreenstackUnitTests: Don't recompile gtest

Change to use the already-compiled gtest & gtest_main libraries,
instead of compiling them again from source (gtest-all.cc takes ~4s to
compile).

Change-Id: I717df541269d8d50362c8cabd8b9ff7d3240eeb4
Reviewed-on: http://review.couchbase.org/60563
Tested-by: buildbot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Trond Norbye <60edd2ef23891a753f231b0c6f161dc634079a93@gmail.com>

daverigby/kv_engine,couchbase/memcached,owendCB/memcached,daverigby/memcached,couchbase/memcached,owendCB/memcached,daverigby/kv_engine,daverigby/memcached,daverigby/memcached,couchbase/memcached,daverigby/kv_engine,daverigby/memcached,couchbase/memcached,owendCB/memcached,daverigby/kv_engine,owendCB/memcached