mdate-sh: Ensure reproducible time output
This change fixes automake bug#20314.
'mdate-sh' pretty-prints the modification time of a file. But it's
output can vary depending on the timezone of the caller. Someone in
timezone GMT-12 will get a different result (day) than someone in
timezone GMT+12. As this output is also used to create/update stamp
files, which influence the further build process, the build result can
vary.
* lib/mdate-sh: Set 'TZ' to UTC which ensures reproducible output.
* NEWS: Announce bug fix.
Copyright-paperwork-exempt: yes