document wav_hdr. do not initialize its sample rate to 44100 which will be overwritten.
FindOSS: also check presence of <soundcard.h> - some old BSD (and some
new OpenBSD) systems provide it.  no need checking <linux/soundcard.h>,
because Linux does provide <sys/soundcard.h>.  for OSS-enabled builds,
add OSS_INCLUDE_DIR to INCLUDE_DIRECTORIES. adjust wildmidi.c for the
changes.