The CMake TarTest fails with the error
mbsnrtowcs.c:116: __mbsnrtowcs: Assertion
`status == GCONV_OK || status != GCONV_EMPTY_INPUT ||
status == GCONV_ILLEGAL_INPUT || status == GCONV_INCOMPLETE_INPUT ||
status == GCONV_FULL_OUTPUT' failed.
on very old glibc versions. Work around the problem by pretending that
mbsnrtowcs does not exist. Libarchive will fall back to mbrtowc.