Speed up fixincludes.

In my case:
$ rm ./stmp-fixinc ; time make -j16

takes 17 seconds, where I can reduce it easily with the suggested
change. Then I get to 11.2 seconds.

The scripts searches ~2500 folders in my case with total 20K header
files.

fixincludes/ChangeLog:

	* fixinc.in: Use mkdir -p rather that a loop.

fixincludes/fixinc.in

@@ -258,12 +258,10 @@ then echo "All directories (including links to directories):"
      echo $all_dirs
 fi
 
-for file in $all_dirs; do
-  rm -rf $LIB/$file
-  if [ ! -d $LIB/$file ]
-  then mkdir $LIB/$file
-  fi
-done
+cd $LIB
+echo "$all_dirs" | xargs mkdir -p
+cd ..
+
 mkdir $LIB/root
 
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