build-sys: support local ./configure arguments

I often want to use the awesome "./autogen.sh [cmd]" arguments, but have
to append some custom ./configure options. For now, I always had to edit
autogen.sh manually, or copy the full commands out of it and run it
myself.

As I think this is super annoying, this commit adds support for
".config.args" files in $topdir. If it exists, any content is just
appended to $args, thus to any ./configure invokation of autogen.sh.

Maybe autotools provide something similar out-of-the-box. In that case,
feel free to revert this and lemme know!