build: Make sure historical release notes end up in distributions

Adds the historical release notes to release tarballs and windows
installers.
Update the address for downloading libpng

The FTP server what we get libpng from only keeps the latest version in its main folder. Older versions are in the "history" folder. Apparently version 1.6.9 has been released, so 1.6.8 has moved to the history folder.
Merge pull request #3635

12edce5 Update the address for downloading libpng (Micha)
Merge pull request #3608

de21e8f build: Make sure historical release notes end up in distributions (Wladimir J. van der Laan)