Since on regular compiles the version information is not critical, it's
better to only update it on make dist; furthermore, there it is indeed
needed to regenerate it automatically, without requiring the use of make
maintainerclean beforehand.

This patch simply adds a new phony target that forces regeneration as a
prerequisite for the dist rule.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>