Merge branch 'gcc-min-version-5.1' (make gcc-5.1 the minimum version)

Merge patch series from Nick Desaulniers to update the minimum gcc
version to 5.1.

This is some of the left-overs from the merge window that I didn't want
to deal with yesterday, so it comes in after -rc1 but was sent before.

Gcc-4.9 support has been an annoyance for some time, and with -Werror I
had the choice of applying a fairly big patch from Kees Cook to remove a
fair number of initializer warnings (still leaving some), or this patch
series from Nick that just removes the source of the problem.

The initializer cleanups might still be worth it regardless, but
honestly, I preferred just tackling the problem with gcc-4.9 head-on.
We've been more aggressiuve about no longer having to care about
compilers that were released a long time ago, and I think it's been a
good thing.

I added a couple of patches on top to sort out a few left-overs now that
we no longer support gcc-4.x.

As noted by Arnd, as a result of this minimum compiler ...