RFR: JDK-8244592 Start supporting SOURCE_DATE_EPOCH

[Magnus Ihse Bursie]

As a part of the effort of creating a reproducible build, the build 
system should know about SOURCE_DATE_EPOCH and allow it to be set using 
different methods.

This fix is needed to unblock JDK-8241616. With this patch, the 
SOURCE_DATE_EPOCH variable will be available in the environment during 
the build.

I have added two options, --with-source-date and 
--enable-reproducible-build. The former controls what value 
SOURCE_DATE_EPOCH will get. The latter is a currently just a 
placeholder, but my intention is to continue adding functionality for 
making reproducible builds. (The reason that you might not always want 
to have reproducible builds enabled is that, for instance, one thing 
reproducible builds will do is set the timestamp for all created files, 
and this is not generally a good idea.) The idea is still that build 
changes that result in reproducible builds should be enabled always, if 
there is no downside to them.

If --with-source-date is specified on the command line, 
--enable-reproducible-build will be turned on by default, otherwise it 
will be turned off.

The --with-source-date option takes either a keyword option, an integer 
option, or a date/time string.
* If "updated" is specified (this is also the default), 
SOURCE_DATE_EPOCH will be set to the time when make is run.
* If "current" is specified, the value is "hard-coded" to the time when 
configure was run.
* If "version" is specified, the value is from DEFAULT_VERSION_DATE in 
make/autoconf/version-numbers is used. (This is probably not as good an 
idea as it might sound, but it can be used as a simple way to get the 
same value between different runs/points in time of the source code, but 
for the same version.)
* If an integer is used, that very number is used as the epoch-based 
timestamp.
* And finally, if an ISO-8059 date string (like "2020-05-07") is used, 
it will be converted to a timestamp. (On systems with GNU date, the 
parsing of the string is rather forgiving, on other systems (read: 
macOS) it will be more strict.)

Finally, this patch carries two unrelated changes.
* In make/autoconf/jdk-version.m4, I noticed an error with the bash 
regexp (which I used as starting point for parsing the timestamp 
number). It would have accepted e.g. "123banana" as a number, but not 
000 (even though 001 was accepted).
* In UTIL_ARG_ENABLE, I added a new argument IF_NOT_GIVEN. In the end, I 
did not need this, but I still think the functionality is worth keeping.

Bug: https://bugs.openjdk.java.net/browse/JDK-8244592
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8244592-add-SOURCE_DATE_EPOCH/webrev.01

/Magnus