On systems where err.h is not provided (e.g. MSVC) configurator must
provide its own definition.  The err module can not be used by
configurator due to its dependency on config.h, so the relevant source
is copied into configurator with the minimum changes necessary.

Changes since v2:
- Use the CCAN err module sources instead of musl libc sources to avoid
  introducing another implementation of these functions.
- Prefix err.h functions with "c12r_" to avoid name conflicts and
  "static declaration follows non-static" errors.
Signed-off-by: Kevin Locke <kevin@kevinlocke.name>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>