Add cmake flag to enable reading and verifying of VERSION file

Some Gpg4win-based distributions of Kleopatra include a signed VERSION
file for customizing the about data and some other information shown by
Kleopatra (e.g. a version number in the status line). This feature flag
allows those distributions to enable reading the VERSION file explicitly
instead of implicitly relying on the presence of the file.

• GnuPG-bug-id: T8245