Mute warnings that we should use _strdup

Otherwise the compiler emits tons of warnings that we
should be using _strdup instead of strdup.

Change-Id: I09c83f3e9c91293fabc46f5fc45ecd9d3914745f
Reviewed-on: http://review.couchbase.org/32723
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>