Add audio output driver plugins.

This allows to detect supported audio output drivers at runtime.
Support is not complete yet : it hasn't been tested on other platforms
than osx yet, and static plugins are not implemented.
Make plugins path and extension configurable at build-time.

Only try to load files with the right extension.
Also add clean and install rules for plugins.
Move test running in a separate function.
Improve Makefile a bit.

Add a mrproper target to clean configuration as well as build
intermediates, fix plugin.mk to use simple variables, and add
all/clean/install/mrproper rule to PHONY.
Use platform files to determine compilation flags of modules
Fix clean and install targets when building multiple plugins