enable support for using the system portaudio library. The latter
also corrects a long standing bug that prevented using config.status
to regenerate src/Makefile.
As a result of the above, I've taken Benjamin's *_LOCAL_CONFIGURE_ARGS
idea and propagated its usage to the other place where ac_configure_args
was being set directly.
Removed old stuff from lib-src/Makefile.in and made the library locations
for portaudio and portmixer like the rest (a symlink in lib-src).
Reworked portmixer's configure.ac to properly detect host apis.
