…th other carve-outs

Previously, we would issue an #error when using a header that requires
threading support or localization support in a configuration where that
is disabled. This is unlike what we do for all the other carve outs
like no-filesystem, no-wide-characters or no-random-device. Instead of
issuing an #error, we normally just remove the problematic parts of the
header.

This patch makes the handling of no-localization and no-threads consistent
with the other carve-outs. I dislike the fact that users won't get an
explicit error message when trying to use e.g. ios in a build that doesn't
support localization, but I think it is better to handle things consistently.
Note that besides the consistency argument, the #error approach doesn't
really work anyways since it would break down if we moved towards assuming
the C locale only in the no-localization mode.