configure: colorize warning messages

This patch introduces color warning messages (yellow).
They are enabled whenever available.
Tested with zsh, bash, and sh (bash under --posix).

Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

configure

@@ -416,6 +416,14 @@ EOF
 }
 
 quotes='""'
+if test -t 1; then
+    ncolors=$(tput colors)
+    if test -n "$ncolors" && test $ncolors -ge 8; then
+        bold_color=$(tput bold)
+        warn_color=$(tput setaf 3)
+        reset_color=$(tput sgr0)
+    fi
+fi
 
 log(){
     echo "$@" >> $logfile
@@ -6201,7 +6209,7 @@ echo "#endif /* AVUTIL_AVCONFIG_H */" >> $TMPH
 cp_if_changed $TMPH libavutil/avconfig.h
 
 if test -n "$WARNINGS"; then
-    printf "\n$WARNINGS"
+    printf "\n%s%s$WARNINGS%s" "$warn_color" "$bold_color" "$reset_color"
     enabled fatal_warnings && exit 1
 fi