Introduce FFMPEG_FORCE_NOCOLOR, with the same meaning and use of

NO_COLOR (but properly prefixed for avoiding namespace conflicts).

Originally committed as revision 25899 to svn://svn.ffmpeg.org/ffmpeg/trunk

doc/fftools-common-opts.texi

@@ -84,7 +84,8 @@ Set the logging level used by the library.
 
 By default the program logs to stderr, if coloring is supported by the
 terminal, colors are used to mark errors and warnings. Log coloring
-can be disabled setting the environment variable @env{NO_COLOR}, or can
-be forced setting the environment variable @env{FFMPEG_FORCE_COLOR}.
+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}.
 
 @end table

libavutil/log.c

@@ -62,9 +62,10 @@ static void colored_fputs(int level, const char *str){
             background = attr_orig & 0xF0;
         }
 #elif HAVE_ISATTY
-        use_color= !getenv("NO_COLOR") && (getenv("TERM") && isatty(2) || getenv("FFMPEG_FORCE_COLOR"));
+        use_color= !getenv("NO_COLOR") && !getenv("FFMPEG_FORCE_NOCOLOR") &&
+            (getenv("TERM") && isatty(2) || getenv("FFMPEG_FORCE_COLOR"));
 #else
-        use_color= getenv("FFMPEG_FORCE_COLOR") && !getenv("NO_COLOR");
+        use_color= getenv("FFMPEG_FORCE_COLOR") && !getenv("NO_COLOR") && !getenv("FFMPEG_FORCE_NOCOLOR");
 #endif
     }