Avoid a crash on launch with openal 1.24.0
BTS entry: https://bugs.debian.org/1087990
The patches are cherry-picked from upstream: https://gitea.wildfiregames.com/0ad/0ad/pulls/7234
They can be dropped when 0ad 0.0.27 is packaged.
In addition to this fix, debian/rules is tweaked to honour the parallel
value passed to DEB_BUILD_OPTIONS. That should make this package easier to build on systems that do not have a lot of RAM available.