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.

(cherry picked from commit d9832bfef2414b1793714e006106d8c22582043e)

• GnuPG-bug-id: T8245