I applied a change

        * configure.in (CC_FOR_BUILD): Use AC_ARG_VAR.

This gets CC_FOR_BUILD described in ./configure --help, and preserves
its value across reconfigures.

Unless anyone is very attached to --with-cc-for-build, I'd suggest
removing that option and just leave the CC_FOR_BUILD variable.  The
latter is moderately standard (eg. config.guess uses it), and an extra
guile specific option won't really add much.