pulseaudio: fix build with orc

The configure script gets orcc from pkg-config, which sets it to /usr/bin/orcc
instead of $(HOST_DIR)/usr/bin/orcc. So set the path explicitly instead of
relying on pkg-config.

While we're at it, also add an explicit enable/disable to configure.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>