Deprecate the use of the NO_COLOR environment variable.

Originally committed as revision 25900 to svn://svn.ffmpeg.org/ffmpeg/trunk

doc/fftools-common-opts.texi

@@ -87,5 +87,7 @@ terminal, colors are used to mark errors and warnings. Log coloring
 can be disabled setting the environment variable
 @env{FFMPEG_FORCE_NOCOLOR} or @env{NO_COLOR}, or can be forced setting
 the environment variable @env{FFMPEG_FORCE_COLOR}.
+The use of the environment variable @env{NO_COLOR} is deprecated and
+will be dropped in a following FFmpeg version.
 
 @end table