Instead of including the IDLs directly, define a local IDL that #includes them,
with XAUDIO2_VER defined, and include that generated header.

Get rid of compat.c, and use XAUDIO2_VER to modify the code in the other source
files.

Build the tests for both xaudio2_7 and xaudio2_8 using PARENTSRC, and use
XAUDIO2_VER to select between them. This mirrors the approach taken for
d3dcompiler, and makes it easier to test more xaudio2 versions in the future.