RFR: 8278766: Enable OpenJDK build support for reproducible jars and jmods using --date [v4]

[Andrew Leonard]

On Fri, 17 Dec 2021 20:04:20 GMT, Andrew Leonard <aleonard at openjdk.org> wrote:

>> make/common/MakeBase.gmk line 148:
>> 
>>> 146:   ifeq ($(SOURCE_DATE_ISO_8601), )
>>> 147:     # GNU date format did not work, try BSD date options
>>> 148:     SOURCE_DATE_ISO_8601 := $(shell $(DATE) -u -j -f "%s" "$(SOURCE_DATE_EPOCH)" +"%Y-%m-%dT%H:%M:%SZ" 2> /dev/null)
>> 
>> Could we maybe figure out the date command line to use in configure instead to avoid the trial and error at build time?
>
> I based this trial and error based on util.m4 code and copied the method, so yes I can store the result from then...

Added IS_GNU_DATE variable

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PR: https://git.openjdk.java.net/jdk/pull/6878