pkg-config alsa --cflags returns -I/usr/include/alsa which is wrong, as you are supposed to include alsa in userspace via #include <alsa/asoundlib.h> So alsa --cflags should either return nothing or -I/usr/include. Of course above "works" with gentoo, as alsa headers are installed in standard location, so they are found via standard include. I think this is rather an upstream bug, right?
Probably neither upstream.