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.

See https://devmanual.gentoo.org/ebuild-writing/common-mistakes/index.html#-werror-compiler-flag-not-removed

Reviewed-by: Eric Anholt <eric@anholt.net>
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