build: Add new gcc error flags

build: Add new gcc error flags

These flags check for code that we don't want to write, so turn them
into error flags.

Variable length arrays should be replaced by malloc() - or explicit
alloca() calls if you know what you're doing.

Implicit fallthrough should be replaced by explicit fallthrough with the
usage of G_GNU_FALLTHROUGH.

This work inspired by Kees Cook's LCA2019 talk:
https://www.youtube.com/watch?v=FY9SbqTO5GQ
http://outflux.net/slides/2019/lca/danger.pdf