Fix libva related compile errors

- Make h264.c compile with recent libva version by including va_compat.h
- Only enable libva if libva-x11 is installed
- Modified configure help text
  Previous help text suggested libva was only build when --with-libva
  was specified, while actual behavior is to build it by default.


WARNING: THIS CODE IS UNTESTED, lacking a h.264 capable VNC server
Strongly suggest you verify that there is actually a h.264 capable VNC server out there, and test this patch actually works beyond compiling, before merging.

Also no attempt is made to support platforms not using X11