CMake: Allow COUCHBASE_MEMORY_ALLOCATOR=system

Allow the system default malloc implementation to be selected by
CouchbaseMemoryAllocator.cmake

Example usage:

    make EXTRA_CMAKE_OPTIONS="-D COUCHBASE_MEMORY_ALLOCATOR=system"

Change-Id: I7b9e859bc39eb7012ae0253edbc96e2fc37cb922
Reviewed-on: http://review.couchbase.org/39484
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Dave Rigby <daver@couchbase.com>
Reviewed-by: Chris Hillery <ceej@couchbase.com>