Replace non-backward-compatible `--eval' null input w/ shell echo pipe.

Replace non-backward-compatible `--eval' null input w/ shell echo pipe.

Specifically, double dashes are preferred in env var default values
but they should be avoided in explicit command lines since they die
when the bootstrap ${SCHEME_COMPILER} predates the double dash syntax.

This really just substitutes one hack for another but at least it now
works for release 7.6 as well as 7.7 (so you can now build the latter
from CVS using the former to bootstrap from 7.6 into 7.7).