allow various x86 models with --tune

Originally committed as revision 4230 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -691,8 +691,11 @@ if test $tune != "generic"; then
 	    TUNECPU=ppc970
             POWERPCMODE="64bits"
 	;;
+	i[3456]86|pentium|pentiumpro|pentium-mmx|pentium[234]|prescott|k6|k6-[23]|athlon|athlon-tbird|athlon-4|athlon-[mx]p|winchip-c6|winchip2|c3|nocona)
+	    CFLAGS="$CFLAGS -march=$tune"
+	;;
 	*)
-	echo "WARNING: unknown CPU "$tune", ignored"
+	echo "WARNING: unknown CPU \"$tune\", ignored"
 	;;
     esac
 fi