Allow each module to override the erlc flags

Change-Id: I6ace945abe038087561c707f07e5594e7b5cff6d
Reviewed-on: http://review.couchbase.org/34926
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>