[New] devel/fmt

Hello,

I have here a new port for the fmt library for C++.

NOTE: For bonus confusion, there's already a devel/libfmt. Please note
these are *not* the same software. (suggestions on naming appreciated)

fmt (https://github.com/fmtlib/fmt) is a formatting library that
provides an alternative to C++ iostreams and C stdio. It's used quite
widely nowadays, it's sort of like the various "pretty printing"
libraries you see in Python.

I have built this on my -current machine (amd64). The package builds and
seems to work. Portcheck reports clean.

Two caveats:

- This is their latest release. However, it seems although they provide
build options to enable -Werror and -pedantic, they apparently don't
actually test it as one of the tests won't build. They appear to have
noticed this and fixed it after this release was cut. As a result,
I've backported their fix so we can compile with the most stringent
options enabled.

- The test suite passes only 15/16 tests. The single failure appears to
be a complaint over the wording of an error message it's expecting -
the way they're testing this stuff is somewhat strange. As far as I
can tell, this doesn't actually matter for anything. Since everything
else passes it seems things are working as intended. I will
investigate this further and send a patch upstream if required.

Suggestions greatly appreciated.

Thanks,

Ash