doc/build.info was essentially generated on the fly while running
Configure, something that takes a huge amount of time on slower file
systems (such as Windows).
Instead, we generate it with 'make update', saving the user from
having to wait for too long, at the small price for developers to have
to run 'make update' whenever they write a new manual file.
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/14269)
