Suggestion for the latest "fix" to ffmpeg

[Terry Chan]

I would suggest that the POST_INSTALL fix that was applied to ffmpeg be moved
to a PRE_BUILD. Removal of out-dated source code should be done in the
PRE_BUILD and new code should be fetched at that time.

The current method removes the source code immediately after the module has
been compiled. This precludes users and devs from actually browsing the source
code should the need arise without first running another fetch.                                                                

Removing the source code after a compile also precludes any chance of
debugging or patch submissions.

Terry Chan