Build with -Wshadow

[Quuxplusone]

Consider fixing the few places that -Wshadow warns about. I see at least this one, before I added -Wno-shadow and stopped worrying about it. It would be nice not to have to add -Wno-shadow just for this.

src/tz.cpp: In lambda function:
src/tz.cpp:670:42: error: declaration of 'm' shadows a previous local [-Werror=shadow]
  670 |                  [&s](std::string const& m)
      |                       ~~~~~~~~~~~~~~~~~~~^
src/tz.cpp:669:10: note: shadowed declaration is here
  669 |     auto m = std::find_if(std::begin(month_names), std::end(month_names),
      |          ^

I recall that 3.0.1 had many more instances — something about a bunch of ifstreams named in or is — but I forget the details.