Make the Unix build process more repeatable

Make the Unix build process more repeatable

Before this change all manpages would contain the date when pod2man was
run. This resulted in outputs that differed between builds--or
potentially across a single build if the host clock "ticked" to the next
day when the build was being run.

This commit modifies the manpage generation process as follows:
- The date all manpages were generated will be normalized to a single
  date.
- The release date specified in `VERSION.dat` is used instead of the
  date/time when `pod2man` was executed OR--in the event a date isn't
  specified in `VERSION.dat`--the time when the Makefiles were last
  regenerated.

Embedding a consistent date into the generated manpages helps ensure that
the build process as a whole is more repeatable and helps ensure that
release versions of OpenSSL create artifacts consistent with the date
that the official release was cut.

Co-authored-by: Richard Levitte <levitte@openssl.org>
Signed-off-by: Enji Cooper <yaneurabeya@gmail.com>
Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/28449)

(cherry picked from commit 2c0c9c83b292fdba001d968a8219db4083294003)