Update build-aux tools to 1.13.1

git-svn-id: https://mingw-w64.svn.sourceforge.net/svnroot/mingw-w64/trunk@5831 4407c894-4637-0410-b4f5-ada5f102cad1
Use find's -delete option instead of calling rm directly

git-svn-id: https://mingw-w64.svn.sourceforge.net/svnroot/mingw-w64/trunk@5832 4407c894-4637-0410-b4f5-ada5f102cad1
Update comments, remove TODO

git-svn-id: https://mingw-w64.svn.sourceforge.net/svnroot/mingw-w64/trunk@5833 4407c894-4637-0410-b4f5-ada5f102cad1