Bug#782294: asciidoc: make timestamps reproducible adding the flag --use-utc-timezone

[Anders Kaseorg]

Now that we have SOURCE_DATE_EPOCH, consider this PR as a replacement.  It 
allows software that builds with AsciiDoc to build reproducibly without 
being patched to pass additional flags:

https://github.com/asciidoc/asciidoc/pull/106

Anders