error.h: Do not circularly depend on avutil.h
configure: Explicitly disable w32threads if the test for it fails

This avoids false positive enabling of w32threads if explicitly
requested on the command line, but dependencies are missing.
mms: Remove non-utf8 characters

Certain softwares get badly confused.

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>