Dealing with version.texi and mdate-sh

[Eric Dorland]

Hi folks,

I was trying to fix some unreproduciblity issues with automake and the
problem of version.texi came to my attention and I haven't seen it
come up before, but let me know if I just couldn't find it.

According to
https://www.gnu.org/software/automake/manual/html_node/Texinfo.html,
if your .texi includes version.texi it will generate version.texi
based on the output of mdate-sh on the .texi file (aka whatever the
modification date of the file is). Obviously that's bad for build
reproducibility. 

My thinking on how to fix this would be to add a flag to mdate-sh to
use SOURCE_DATE_EPOCH if it's available and make automake use that
flag when generating version.texi. Is there a better approach? Should
the unpacked source package just have all of the files modification
dates set to SOURCE_DATE_EPOCH?

-- 
Eric Dorland <eric at kuroneko.ca>
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