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>