Fix: avoid assignment discards 'const' qualifier warning

Untested. Please test.
See #2063
Update: Make stuff compile with new version of mingw.

This will make stuff working with newer versions of mingw.
Will likely break older versions. Untested. Also requires updates
to m4/ and common/. Those are not yet commited as supid git can not
do that...
Revert "Fix: avoid assignment discards 'const' qualifier warning"

This reverts commit 0dbabcc44c141d0018a6eb6b6d3c78cc6e16491c.
Breaks other OpenSSL versions.