Zuo: show `CC_FOR_BUILD` in `--help` and infer from `--host`

[LiberalArtist]

The first commit just tweaks the help output from Zuo's configure.ac. The second uses AX_PROG_CC_FOR_BUILD from the Autoconf archive to infer those values, so you can do ./configure --host=powerpc64le-linux-gnu rather than ./configure --host=powerpc64le-linux-gnu CC_FOR_BUILD=gcc. If the extra M4 file doesn't seem like too much, I think it might be even more useful elsewhere.

[mflatt]

The first of the two commits looks like a clear improvement.

For the second commit, I'm still reluctant to pull in the extra support for configure to define CC_FOR_BUILD. Also, it doesn't seem like a good idea for zuo sources to depend on things inside the Racket source tree but outside the zuo directory. How bad is requiring CC_FOR_BUILD to be specified? Is it common that it's inferred automatically?

[LiberalArtist]

I've dropped the second commit. Supplying `CC_FOR_BUILD' isn't causing any difficulty for me here, and I agree that keeping Zuo as self-contained as reasonably possible seems good.

[mflatt]

Thanks!