notes on building 3.20

[Matthias Clasen <matthias clasen gmail com>]

Hey, some quick observations from building the 3.20.0 moduleset with
gcc 6 on Fedora 24:

gcc got a lot better. That means it has a lot more warnings. The ones
that affect multiple modules are -Werror=format-nonliteral and
-Werror=format-y2k. Both of cause build failures all over the place.

Please make an effort to fix these in your code. At least the
format-nonliteral warning often indicates actual, exploitable security
holes.

Matthias