Allow for specifying alternate cmake

This allows to specify a different installation of cmake to use, for
systems which have an older version of cmake installed. Using the `make
CMAKE=/path/to/cmake-bin` syntax one may specify an alternate version of
CMake

Change-Id: I6c9338286882d6755f289a98ea1cc01d4cdf1275
Reviewed-on: http://review.couchbase.org/37609
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>