Hack compiler flags to silence warnings on gcc 4.8

GCC changed its default behaviour when reporting a number of different
kinds of warnings. We don't want to fix these in the bugfix branch
because even where it is possible it looks too invasive. We plan to
address these issues in a less heavy-handed manner in master branch.

Note when merging to master, do not include this patch.

Change-Id: I14f00357195424dd538aca4b319e5c2c120c691d