bug#18714: [mdate-sh] use specific timezone when producing input
Currently the mdate-sh is timezone sensitive, which means that if a file is not modified but timezone modified then output differ.
This triggers rebuild of targets such as texinfo targets even if no change was performed.
For example, build machine with timezone X created tarball, then another build machine with timezone Y build the package out of the tarball, while no file was actually changed, the mdate-sh triggers rebuild per different timezone.
A possible solution is to return the date in UTC timezone regardless of system timezone.