ffmpeg needs Xext lib (if found)
ffmpeg needs Xext lib (if found)

* use script specifies this library (if found)
* ffmpeg may have been built on a system where configure detected and built against Xext library
* issue https://github.com/smanders/externpro/issues/212#issue-356002836
* perhaps the `--disable-xlib` in the previous commit may avoid the need to add Xext to the link line?
  * I just don't have a system to test this on, so the less risky thing is just to add it to the link line

smanders/externpro