build: don't default to -Werror

build: don't default to -Werror

It seems like a poor default for various downstream toolchains. We can't
anticipate the compiler warnings for future compiler versions and older
ones are prone to false positives. Also, older gdbus-codegen is known
to generate code that triggers compiler warnings.

Let's keep it enabled for maintainer builds and distcheck so that we're
sure a tool chain that builds releases without warnings exists.