Only set CMAKE_MODULE_PATH if not set

The tlm sets the CMAKE_MODULE_PATH. If a geocouch build is run without
the tlm (e.g. for the special build for the unit tests) then the module
path is set.

Change-Id: I439eaef8ad3a6ffed447b1dbf83fa2a8263f3756
Reviewed-on: http://review.couchbase.org/42290
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Volker Mische <volker.mische@gmail.com>