FMT_USE_TRAILING_RETURN not defined on clang, causes warnings

[eliaskosunen]

Please see this CE snippet: https://godbolt.org/g/CVWCax

As a solution, I propose either:

• adding a macro FMT_HAS_CXX11, that checks for __cplusplus and/or compiler versions.
• defining FMT_HAS_GXX_CXX11 as a non-zero value on clang as well

I'm not proposing:

• defining FMT_USE_TRAILING_RETURN to zero instead of not defining it at all,

as this would lead to trailing returns to not be used on clang.

Once we reach a solution, I can submit a PR.

[eliaskosunen]

As an aside, to avoid these kinds of problems in the future, Travis CI could test with more compilers. At the moment, only g++-6 is used on Linux. Clang with -Weverything -Wno-c++98-compat -Wno-c++98-compat-pedantic -Wno-padded -Wno-exit-time-destructors yields, in my opinion, much better warnings than g++.

[vitaut]

I think the simplest fix is to define FMT_USE_TRAILING_RETURN whenever __has_feature(cxx_trailing_return) is defined (in addition to the existing conditions). PR is welcome!