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pulseaudio: uses fork(), not available on noMMU platforms

Even though pulseaudio has some HAVE_FORK compile-time conditionals to
avoid using fork(), those parts of pulseaudio are not using it. Since
using pulseaudio on !MMU platforms is fairly unlikely, we simply
disable it when no MMU is available, and propagate this dependency to
the appropriate locations.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>