Hard-code the include paths

Instead of dynamically building the include paths from the COUCH_SRC
environment variable, it is not hard-coded. The reason is that setting
environment variables from within CMake is not straight forward.

Change-Id: Ib34d556450d73a601cf16848e5f017cb9b01ca75
Reviewed-on: http://review.couchbase.org/40173
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Volker Mische <volker.mische@gmail.com>