Copy the latest gnulib bootstrap, which runs autoreconf and
generates po/Makevars for us. Other improvements include some
improved prerequisite tool checking.
This also fixes a bug in the .pot files, regarding the copyright holder.
* bootstrap: Update to version in .gnulib/build-aux.
* bootstrap.conf (MSGID_BUGS_ADDRESS, COPYRIGHT_HOLDER, SKIP_PO)
(gnulib_mk, ACLOCAL, bootstrap_epilogue): Provide overrides.
* autogen.sh (autoreconf): Avoid redundant autoreconf if bootstrap
was run.
* po/Makevars: Delete, now that bootstrap creates it.
* po/.gitignore: Update.
Borrow ideas from gnulib/build-aux/bootstrap, in order to factor the
specifics of libvirt into bootstrap.conf, while allowing future
upgrades of bootstrap to happen with less effort.
* bootstrap (gnulib_tool): Update invocation to be closer to
gnulib's version. Move libvirt specifics...
* bootstrap.conf: ...into new file.
