C++14 build fixes for older gcc #2333

[kslattery]

@gabime I'm unable to test that the changes for __has_include in bin_to_hex.h work for @aengusjiang, but the approach I've followed matches the recommended pattern in the gcc documentation for __has_include.

I have tested the rest of the changes on my machine with gcc version gcc (Ubuntu 5.5.0-12ubuntu1~14.04) 5.5.0 20171010. I get the overloaded formatter used as seen in the output from the example program:

[2022-05-11 18:22:25.008] [info] user defined type: [my_type i=14]

The problem seems to be caused by this clause in the namespace specification (emphasis added):

To reopen an existing namespace (formally, to be an extension-namespace-definition), the lookup for the identifier used in the namespace definition must resolve to a namespace name (not a namespace alias), that was declared as a member of the enclosing namespace or of an inline namespace within an enclosing namespace.

The approach I've taken here is the same approach used within fmt for the FMT_BEGIN_NAMESPACE macro defined in include/spdlog/fmt/bundled/core.h.

[gabime]

Thanks a lot. I think the has_include fix is ok, but the namespace define can be replaced with an example instead. I prefer not to add yet another ifdef to the code if possible.

[kslattery]

Makes sense. I wasn't sure what you'd want to do. The problem with just using namespace fmt in the example code is that the example will then not work if using the std formatter. I'd suggest putting something like this in the example to make it work either way:

#ifdef SPDLOG_USE_STD_FORMAT
    namespace std {
#else 
    namespace fmt {
#endif
template<>
struct formatter<my_type> : formatter<std::string>
...
}. // end of namespace

[gabime]

I'd suggest putting something like this in the example to make it work either way

Yes. Thank you.

[kslattery]

Ok, do you want me to create a new commit with those changes? Or do you want to just take it from here?

[gabime]

Please do.

[kslattery]

@gabime If you don't like the comment I added, let me know what, if anything, you'd like in its place.

[gabime]

Thanks @kslattery

[anhns26693]

Thanks @kslattery very much