Making javadoc reproducible

[Gábor Boskovits]

Hello guix,

I've tracked down the javadoc timestamp problem.
There is a command line flag for javadoc (notimestamp), that disables
generating the comment in the docs that contains the timestamp.
Currently I see two ways forward:
1. Track down the calls to javadoc, and add the flag to all calls.
2. Write a simple patch to make javadoc behave as if notimestamp was
specified, whenever
SOURCE_DATE_EPOCH is defined.
I do not think, that the patch produced by 2 is upstreamable, but it
seems much less work. WDYT?