This is intended to prevent the same != execution in a sub-make for an
already-known value. Similar efforts have gone into ports over the
years.
With 'make -n -n buildworld' you can see a 'cc --version' ran for just
about every Makefile even though CC is unchanged.
For ports at least we just .export the value (using .MAKEFLAGS due to
needing to support fmake). We can't just .export here though since
CC might change in the sub-make, such as if we called it with
${LIB32WMAKE} from Makefile.inc1. So I've cached the value based
on what CC was, along with MACHINE since it seems relevant. The value
is picked up in the child only if these values match.
This uses .export-env so the value can be unset after, otherwise
.export binds to the real var post-exporting.