Don't assume they're run from the directory containing them, especially
as this is not how they are shown to be invoked in the release
instructions.
Now running ./build/tools/pre-release.sh works too.
Update dates in various files before the release automatically.
Also automatically update SHA-1 of the release files both before the
release (to zero them) and afterwards (to use the correct values).
Closes https://github.com/wxWidgets/wxWidgets/pull/443
