build: downgrade maybe-uninitialized from an error to a warning

maybe-uninitialized has false positives in gcc 4.8 due to
Boost ticket #9134 https://svn.boost.org/trac/boost/ticket/9134

refs #1560

Change-Id: I44e2456b31f3a102d4a447da0dc81c23efb31f3b