utils: Change `substitute*' to allow iteration over several matches.

utils: Change `substitute*' to allow iteration over several matches.

* guix/build/utils.scm (substitute): Do not pass the OUT to PROC; use
  `list-matches' instead of `regexp-exec' and pass a list of matches to
  PROC. Expect PROC to return a string, and output that.  Fold over
  RX+PROC in order.  Use `(read-line p 'concat)' to include the trailing
  delimiter in LINE.
  (substitute*): Produce code to iterate over the matches, and return a
  string, which includes anything from the original line that's in
  between matches.

* distro/base.scm (gcc-4.7, glibc): Adjust accordingly: remove use
  of (ice-9 regex) and `regexp-substitute/global'; return a string.