[winswitch] Centos libvpx.so.4 dependency

[Bob Tennent]

 >|> --> Processing Dependency: libvpx.so.4()(64bit) for package:
 >|> ffmpeg-xpra-3.1.2-1.el7_2.x86_64
 >|>
 >|> --> Processing Dependency: libvpx.so.4 for package:
 >|> ffmpeg-xpra-3.1.2-1.el6_8.i686

 >|I guess you're having some dependency problems on the latest CentOS?
 >|Can you please clarify what the problem is? Is the required libvpx
 >|library version not available in those repos?

Yes. ffmeg-xpra requires libvpx.so.4, which isn't available.
On Centos 7 I've built and installed libvpx-1.6.0 which
provides it and then xpra-0.17.5 installs ok.