Following
https://github.com/mozilla/cubeb/issues/658#issuecomment-955998734, the
speex library is now handled like a normal dependency: cubeb will link
against the system version if available, and fall back to the bundled
one if not.
I've also added a BUNDLE_SPEEX option, so that you can force the use of
the bundled library if needed (e.g. creating a standalone libcubeb on a
system where libspeex is available).
I also had to move the bundled library to a separate folder. As `src` is
always added as an include path, the headers in `src/speex` would
conflict with system headers. And it also clears the relationship
between cubeb and speex. I choose the "subprojects" name to follow the
Meson convention, since CMake does not have one. A bit OT, but if you're
curious you can see their rationale here:
https://mesonbuild.com/Subprojects.html#why-must-all-subprojects-be-inside-a-single-directory
Lastly, I added cubeb_log.cpp to the list of sources of test_resampler,
as I was getting linking errors when building with BUILD_SHARED_LIBS=true
Fixes#658
