start work on switchable cubeb backends.
pulse: build fix.
allow building more than one backend at once.
rework backend detection in configure.
audiounit, audioqueue, sndio: build fixes.
winmm: build fixes.
directsound: (re)disable build until it can be fixed.
clean up backend initialization logic.
remove pedantic from CFLAGS.
pulse: load pulse DSO and symbols at runtime.
remove pulse DSO from buildtime link flags.
missed renames.  call internal destroy rather than going through vtable.
sndio: build fix.
pulse: dlclose libpulse when destroying context.
make config.h conditional.
directsound,winmm: remove C99 struct initializer syntax.
remove generic stream_params validation from backend stream_init since it was moved to the shared code.
reorder initialization of audioqueue and directsound.
Merge branch 'switchable-backends'