Cleaning up enabling of FFMPEG.

FFMPEG support was not getting enabled due to buggy logic. Cleaned that up. Now,
when PARAVIEW_ENABLE_FFMPEG is ON, we require VTK's FFMPEG support modules and
enable *.avi support. This replaces VTK_HAS_FFMPEG_SUPPORT with
PARAVIEW_ENABLE_FFMPEG in vtkPVConfig.h.

Change-Id: I047f1143857468542a448d90a0dc58b1be8a0d1a
Cleanup OggTheora support.

ParaView wasn't detecting that OggTheora was enabled when using
system-oggtheora since we were checking for the present of a target which is
only created when not using system OggTheora. This fixes that issue.

Change-Id: I643cb2bd1c18b9abc0e695c856b6786970a0ea5f
Merge topic 'fix_ffmpeg_support' into next

6230007 Cleanup OggTheora support.
b61e99d Cleaning up enabling of FFMPEG.