MB-8042: some platforms need libm

Change-Id: I1d67b69f90dbee803b4ee408d54f64b252e032ec
Reviewed-on: http://review.couchbase.org/25719
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>