Recording TIMESTAMPs in distinfo for reproducible builds work

[Ed Maste]

Baptiste and I have been looking at reproducible builds in the FreeBSD
ports tree, and one thing we'll need is a consistent timestamp that
doesn't change when a port is rebuilt without changes.

We considered a few different ideas, and have settled on experimenting
with the time 'make makesum' is run.

I have a bsd.port.mk change that I'll commit shortly to record the
TIMESTAMP when "make makesum" is run.  I want to do this now so that
this data is collected and stored "for free" along with regular
distfile updates.  This will allow experimentation and development of
reproducible package builds with real data.

For now ports that have no distinfo file, and distfile updates done
without using "make makesum," can just ignore the TIMESTAMP.

-Ed