[Libguestfs] Some fixes for building the Debian package for 1.20.2

I needed to add a few patches to be able to build libguestfs 1.20.2 out of tree. Mostly srcdir/builddir issues -- I think that I had posted something for building the daemon before. As mentioned on IRC, I found that compiling (Ruby bindings) caused config.h file shipped with the Ruby headers to be included rather than ${builddir}/config.h. This can be fixed because the relevant checks can be performed in the extconf.rb script as well. Cheers, -Hilko