[Cin] Attempt at fixing screencapture under NetBSD

Tried to record desktop, found out screencapture does not work :( It seems even ffmpeg fall back to non-shm capture (slower) under NetBSD, so I tried to to change logic in guicast/bccapture.C so if compiled on netBSD it hard disable shm for capture (playback is fine) Please try on Linux and other systems ... Pay attention to screencapture slowdowns!