The following patch allows to specify a required Guile version in
GUILE_PROGS. By default, it requires >= 2.0. The following works:

GUILE_PROGS([2.0])
GUILE_PROGS([2.0.9])

It also works with 1.8.

I am not an expert on m4 macros. I've used "cut" instead of
m4_bregexp. I tried it but couldn't get it right.

In some packages I have implemented procedures which will be available
in the next release of Guile. My plan is to remove these procedures
and add a requirement on a Guile version.

Let me know what you think.

Aleix