Improve autotools, cross compile and Debian build support [ADAPTIVE]

Currently building uade with separate src/build dirs with autotools make distcheck doesn't work. Same for Debian dpkg-buildpackage or cross compile builds.

My use case is using UADE as submodule/subproject in autotools project: https://github.com/mvtiaine/audacious-uade/blob/master/configure.ac#L92