Don't allow ' ' in the preprocessor macro

Change-Id: I8506412bcfdcb30937ffd518394214486cbf2b9b
Reviewed-on: http://review.couchbase.org/35497
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>