bootstrap: specify particular version in buildreq with =x.y.

* gl/build-aux/bootstrap.in (func_check_versions): If the version
number begins with '=' then it must match the installed version of
the named tool exactly.
* gl/doc/bootstrap.texi (buildreq): Document the '=vernum' feature.
* bootstrap: Regenerate.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
options-parser: --version works with 'DO NOT EDIT' preamble again.

* gl/build-aux/options-parser (func_version): Don't quit on first
leading '##' line, otherwise DO NOT edit warnings prevent version
information from being extracted correctly.
* bootstrap: Regenerate.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>