Instead of requiring --enable-compile-warnings or
--enable-compile-warnings=yes, allow any value which is not ‘no’. This
enables compile warnings for --enable-compile-warnings=maximum or
--enable-compile-warnings=error, which are common values for other GNOME
projects. While we don’t change our behaviour for [yes, maximum, error],
at least it means the warnings are enabled now, rather than disabled.

Signed-off-by: Philip Withnall <withnall@endlessm.com>