Add _XOPEN_SOURCE=600 to CPPFLAGS on NetBSD.

This make NetBSD compile out of the box at the cost of a hack, but a
locally contained one that is preferable to the one used in the wild.
Currently the NetBSD FATE box adds -D_NETBSD_SOURCE to CPPFLAGS to work
around broken system headers. Since it is unlikely for NetBSD to fix their
headers, it is better to use a standard flag instead of a system-specific one.

As a sideeffect, this allows getting rid of a local _NETBSD_SOURCE definition.

Originally committed as revision 22486 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -2151,6 +2151,7 @@ case $target_os in
     netbsd)
         oss_indev_extralibs="-lossaudio"
         oss_outdev_extralibs="-lossaudio"
+        add_cppflags -D_XOPEN_SOURCE=600
         ;;
     openbsd)
         enable malloc_aligned

libavdevice/bktr.c

@@ -25,7 +25,6 @@
  */
 
 #define _BSD_SOURCE 1
-#define _NETBSD_SOURCE
 
 #include "libavformat/avformat.h"
 #if HAVE_DEV_BKTR_IOCTL_METEOR_H && HAVE_DEV_BKTR_IOCTL_BT848_H