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

[Hilko Bengen]

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