fix clang warnings

[Lectem]

When building with c++14 and -Weverything, I encountered the following warnings which are now fixed :

• dynamic exception specifications are deprecated [-Wdeprecated]
• 'FMT_USE_OVERRIDE' is not defined, evaluates to 0 [-Wundef]
• definition of implicit copy constructor for 'FormatError' is deprecated because it has a user-declared destructor [-Wdeprecated]

[Lectem]

To make GCC compile while removing the deprecated warning of throw() for clang, I had to introduce a new define FMT_DESTRUCTOR_NOEXCEPT as the header would still use throw() even when exceptions are disabled. It has the same behaviour as FMT_NOEXCEPT for compilers other than GCC.
It seems to be more of a libstdc++11 problem though, I'm not sure if it could pose problems for some versions of clang.

[foonathan]

I am personally not happy with the destructor noexcept workaround (but @vitaut opinion counts).
The problem could be solved more elegantly if we remove the destructor in the exception classes, because the implicitly generated one should be throw() in those cases automatically (IIRC). I'm guessing the destructors are there to avoid the weak vtable warning?

Anyways, I'd merge that PR (and would include it in the 3.0.1 I was just about to release), but I'm waiting on @vitaut's opinion here.

[Lectem]

I wanted to remove the destructors at first too but I suspected they were added for a reason, it would be nice to confirm why.

[vitaut]

Merged with minor changes in a5b9611. Thanks, @Lectem!

@foonathan, I'd prefer to remove dtors too, but as you correctly guessed they were added to prevent -Wweak-vtables warning in clang (b26e76e). Alternative suggestions are welcome.

[vitaut]

Hmm, the fix caused failures on Travis, so I moved it to the https://github.com/fmtlib/fmt/tree/warnings branch for further investigation.

[vitaut]

Merged to master: 8f455c1.