Revert unconditional config.h overwriting, it may cause unnecessary rebuilds.

Originally committed as revision 5020 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -1939,7 +1939,13 @@ for codec in $CODEC_LIST ; do
     echo "CONFIG_`echo $codec | tr a-z A-Z`=yes" >> config.mak
 done
 
-mv -f $TMPH config.h
+# Do not overwrite config.h if unchanged to avoid superfluous rebuilds.
+diff $TMPH config.h >/dev/null 2>&1
+if test $? -ne 0 ; then
+        mv -f $TMPH config.h
+else
+        echo "config.h is unchanged"
+fi
 
 rm -f $TMPO $TMPC $TMPE $TMPS $TMPH