This is a partial revert of 785e61a5.

The _BUILD_SPEEX special case isn't needed in the public headers, and
we don't need '../include' relative paths in the internal files.
Drop a few of the latter altogether where foo.c was including headers
already included by its foo.h.

The OUTSIDE_SPEEX guard isn't needed here anymore either, since that
was only for the resampler which is now in a separate source tree.