Don't assume that isdigit is in std namespace

Change-Id: Ic219afa4a659d743fbadbde16154e32f403433d8
Reviewed-on: http://review.couchbase.org/31194
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>