Remove -Werror from default CFLAGS
The set of warnings emitted are dependent on compiler version, optimization level, CPU architecture, dependency versions, etc. It's not tractable for a code base to be warning free under all circumstances, and it's widely agreed that -Werror should not be enabled by default.
Given that it's trivial for developers to enable -Werror for autotools by settings CFLAGS=-Werror and for meson with -Dwerror=true we should not enable -Werror by default and require builders to disable it.