buildworld fails (missing /usr/share/mk/src.opts.mk)

Hi Warner,

as already reported by Jenkins, HEAD does not build.

Seems that this is caused by src.opts.mk missing in /usr/share/mk
during the cleandir phase. I guess this is kind of a bootstrap
issue - the definitions are looked up in the installed base, not
in the src tree - but did not verify this assumption.

A work-around is to manually install src.opts.mk:

# make -C /usr/src/share/mk install

(which might deserve an UPDATING entry). Falling back on the file
in the src directory might be a better solution ...

Regards, STefan