The following script was applied:
sed ':a;/part of MXE.$/{N;s/\n//;ba}' -i $(git grep -l 'part of MXE')
sed 's/\(part of MXE\).*\(See index.html\)/\1. \2/' -i \
$(git grep -l 'part of MXE.*See index.html')
before='This file is part of MXE. See index.html for further information.'
after='This file is part of MXE. See LICENSE.md for licensing information.'
sed "s/$before/$after/" -i $(git grep -l 'part of MXE')
Then git grep 'index.html for further information' revealed two other files.
One of them was patched manually (patch.mk). Makefile has text
"See index.html for further information" unrelated to licensing.
See https://github.com/mxe/mxe/issues/1500#issuecomment-241340792
Boris Nagaev