app-emulation/libvirt-9999: Perform out-of-tree build

app-emulation/libvirt-9999: Perform out-of-tree build

In preparation to move to meson, we've forbidden in tree builds.
Fortunately, there is a gentoo eclass that handles out-of-tree
builds and very little is needed to use it. However, due to some
automake magic, we have to enable dependency-tracking (which is
disabled as of EAPI 4) because if we don't, then generated
Makefile doesn't precreate all directories under build dir and
the build fails.

Closes: https://github.com/gentoo/gentoo/pull/13752
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Doug Goldstein <cardoe@gentoo.org>