Set the math lib in a variable

Change-Id: I1c79eade133f2d4fd29c7b6f6febcd29b0f736a4
Reviewed-on: http://review.couchbase.org/32863
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>