Allow opt-out from jack, pulseaudio and avcodec dependencies.

Allow opt-out from jack, pulseaudio and avcodec dependencies.

Without this patch the jack, pulseaudio and avcodec discovery was
"automagic", without a way for the user to disable the relative
plugins if the dependencies are installed but the plugin is unwanted.

This patch does not change the default behaviour but allows to opt-out
from the plugins by passing the relative --without option at
./configure time.

Signed-off-by: Diego 'Flameeyes' Pettenò <flameeyes@gmail.com>