Unfortunately that patch to Automake's mdate-sh is not portable, as TZ='UTC' is 
not a portable setting for the TZ environment variable. POSIX says you're 
supposed to use something like TZ='UTC0' instead. Although TZ='UTC' works when 
glibc is used, this is not necessarily true on other POSIX platforms.

I noticed this problem when recent Automake changes were merged into Gnulib, and 
installed the attached patch to the Automake master branch to fix this. Please 
review any other patches you may be using for reproducible builds, and fix them 
to use TZ='UTC0' instead of TZ='UTC'. Thanks.

For reference, here's the POSIX spec for TZ:

http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html#tag_08_03

and look for "TZ".