build: prefer 4Front OSS to native implementations

If sys/soundcard.h is actually linux/soundcard.h then it supports only OSSv3
API. This may happen when OSSLIBDIR == /usr while forgetting to replace
sys/soundcard.h from glibc.

However, after fa620ff waf prefers native implementation which is inferior
on Linux. To fix try making waf prefer oss-audio-4front. It's quite unusual
to have 4Front OSS installed where native implementation is superior, anyway.
Signed-off-by: bugmen0t <@>

Make the false positives path also undef the 4Front define.
Signed-off-by: Stefano Pigozzi <stefano.pigozzi@gmail.com>

Fixes #396
stream: fix clang warning

Good clang catches programming errors. `open(2)` takes `int` not `mode_t`.