build: only check 4Front OSS after passing its CFLAGS
build: unbreak __get_osslibdir__()

- without Utils.* always returns empty string
- subprocess doesn't need extra quoting for sh -c
- "source" is a bash'ism, not in POSIX sh
- most shell commands embed newline at the end
build: run oss_audio.c check with oss cflags as well
build: don't abort if our OSS implementation isn't from 4Front

Original commit was implemented differently by @bugmen0t. The problem here was
that the waf API was called directly, instead of using our own check_cc (which
defaults, among other things, to non mandatory checks).