Keep MEMORY_ALLOCATOR as string instead of bool

As COUCHBASE_MEMORY_ALLOCATOR is a user-specified value it may be
empty (unless the user explicitly set it). Therefore to correctly
conditionalize other CMakefiles keep MEMORY_ALLOCATOR as a string (set
to the in-use allocator) instead of a bool.

Change-Id: Ifa17bf80c022da3d1354b513c690ab7da2b0df95
Reviewed-on: http://review.couchbase.org/43365
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>