do not embed version and timestamp information

[hannesm]

#54 introduced the timestamp date to honor the SOURCE_DATE_EPOCH. This is insufficient if the reproducibility goal is to reproduce under any LOCALE (since the output of date depends on the locale set in your environment).

From #54 Actually, I don't even remember why I chose to have the compilation date as part of the information contained in ocamlgraph. I could simply remove it., this is why I open this PR which removes the timestamp and version information from ocamlgraph.

If you think there's value in version or date informaion, I'm fine adjusting this PR to remove less. I (non-exhaustively) searched for users of Graph.Version. and couldn't find any. I usually install ocamlgraph via opam, and in this case the version information is carried by other means (META) already.

[hannesm]

other possibilities than removing all of it:

• remove date only (keep val version : string)
• modify date to date -Ru (for locale- and timezone-independent output) -- though only -u is specified by posix, not sure how portable -R is (it works on FreeBSD -- Use RFC 2822 date and time output format)
• set LANG/LC_ALL/ ... in Makefile to something default (C?) if SOURCE_DATE_EPOCH is set to avoid locale-dependent output

[backtracking]

I've asked colleagues that are heavy users of OCamlGraph about this. If they do not complain, I'll merge.

[backtracking]

They are not using Version. So I'll merge.