Over time, Fedora and RHEL RPMs have often backported upstream
patches that touched configure.ac and/or Makefile.am; this
necessitates rerunning the autotools for the patch to be effective.
Making this a one-liner spec tweak will make it easier for future
backports to pull patches without having to find all the places
to touch to properly use the autotools.  Meanwhile, there have been
historical instances where an update in the autotools caused FTBFS
situations, so this is not on by default.

* libvirt.spec.in (enable_autotools): New variable, default off.
(BuildRequires): Conditionally add autotools.
(%build): Conditionally use them before configure.
* mingw32-libvirt.spec.in: Likewise.