targets/audio: Allow overriding of ADHD_HEAD (for automated testing).
test/30-audio: Test other ADHD branches (compile-only).
targets/audio: Fix error handling:
 - local VAR="`false`" eats the return status: local declaration must be separated from assignment.
 - function calls in backticks are not called with `-e` in bash: explicitely call return
audio: Put 32/64 compatibility definitions in separate header file; Fix for >=stabilize-5062.