Log Message:
Disable usage of POSIX thread priority functions on NetBSD < 4.99.50
(they are all stubs before that).  This stops wxGTK (and in turn,
Audacity and others) from displaying annoying "Cannot get priority range
for scheduling policy 0" message at startup:

http://mail-index.netbsd.org/tech-pkg/2005/11/22/0020.html
http://mail-index.netbsd.org/netbsd-users/2006/07/02/0004.html
http://mail-index.netbsd.org/netbsd-users/2007/06/28/0002.html

OK by wiz@.