This package should drop -Werror enabled by the buildsystem.
Why? /me is upstream
http://blog.flameeyes.eu/2009/02/future-proof-your-code-dont-use-werror
Sure… but mksh itself does not add -Werror, except for a number of configure tests that require it and will yield wrong results without it. So if you have -Werror somewhere, it comes from Gentoo. (Besides, I'd take flameeyes with a grain of salt.)
-Werror is only used for the configure bits, which is normal. Just in this case the checks are verbose so they end up in the log.