Some simple build simplification patches

[Neil Jerram]

Hi all,

Following are 4 simple (I believe) patches that simplify the build.
Plus the one at the end (#5) that is only very loosely related to this,
because of its mentioning of the $GUILE variable, but I think is a
useful cleanup anyway.

For me, these are part of working towards understanding and simplifying
cross-compilation, and the next step is seeing if we really need the
setting and using (in a few places only) of the $GUILE variable.

Any comments / objections?

Thanks,
    Neil


PS. It's also my first try of using git send-email to send patches to
the Guile list, so here's hoping that that works out...