Make sure ALSA CRAS plugin works with 32-bit applications on x86_64 chroots.

Also remove uneeded dbus dependency (which makes our life easier on Debian `wheezy`).

Ubuntu precise does not provide proper multiarch packages for `libasound2-dev`. Fortunately, we can substitute for `libasound2-dev:i386` by hardcording `ALSALIBDIR` and creating the missing library symlink.

Compile-tested on every single supported Ubuntu/Debian release, on both i386 and x86_64 architecture. Tested on ARM `wheezy` & `precise`.