Similar to the MSC Compiler, the NVidia NVCC compiler also
emits unreachable code warnings when there is a return
statement following an exception.  These changes eliminate
those warnings.

* Squelch MSVC warning exporting subclasses of runtime_error

When compiling {fmt} as a DLL, MSVC complains that we are exporting
classes that inherit from "std::runtime_error", which we are not
exporting.

In this case, it's not really a problem because that symbol is already
exported via the C++ stdlib. So we just add a pragma to silence the
warning.

* Fix compilation with MinGW

Commit 3bc28fcc ("Squelch MSVC warning exporting subclasses of
runtime_error", 2019-11-29) silenced a MSVC warning under. The MinGW
compiler also defines _WIN32, but does not support the "warning" pragma.

Introduce a helper macro to squelch the MSVC warning only when using the
Microsoft compiler.
Signed-off-by: Beat Bolli <dev@drbeat.li>

* Fix compilation with VS2015 (#1450)

VS2015 does not support the __pragma(...) syntax in the midst of a
class declaration, so move it to just before the declaration.