Run asciidoc in UTC timezone for build reproducibility.

asciidoc adds a timestamp at the end of a generated HTML file.
This timestamp is based on the date of the file but it can change
depending on the TZ environment variable.

changes/asciidoc-UTC

@@ -0,0 +1,4 @@
+  o Minor bugfixes (build):
+    - When building manual pages, set the timezone to "UTC", so that the
+      output is reproducible. Fixes bug 19558; bugfix on 0.2.2.9-alpha.
+      Patch from intrigeri.

doc/asciidoc-helper.sh

@@ -19,7 +19,7 @@ if [ "$1" = "html" ]; then
     base=${output%%.html.in}
 
     if [ "$2" != none ]; then
-      "$2" -d manpage -o $output $input;
+      TZ=UTC "$2" -d manpage -o $output $input;
     else
       echo "==================================";
       echo;