Add a way to optionally suppress usage of std::source_location

[franzpoeschel]

The integrated checks for availability of std::source_location seem to not work correctly for Clang++ 11-13, compare this output from our failing CI:

[ 42%] Building CXX object CMakeFiles/picongpu-hostonly.dir/plugins/openPMD/toml.cpp.o
In file included from /builds/hzdr/crp/picongpu/include/picongpu/plugins/openPMD/toml.cpp:38:
In file included from /builds/hzdr/crp/picongpu/thirdParty/toml11/include/toml.hpp:29:
/builds/hzdr/crp/picongpu/thirdParty/toml11/include/toml11/compat.hpp:494:30: error: no type named 'source_location' in namespace 'std'
using source_location = std::source_location;

This PR adds a way to suppress usage of std::source_location:

#define TOML11_DISABLE_SOURCE_LOCATION
#include <toml.hpp>

[franzpoeschel]

@ToruNiina Could you have a look at this PR, please? This is currently blocking us from upgrading to the recent version of toml11.
I think that the failing CI run is unrelated to this, as the PR only adds an additional macro check.

[ax3l]

Hi @ToruNiina, if you have a chance, could we please merge this to ensure one turn off the std::source_location usage?

In some C++20 compilers, this feature is still flaky and we would like to turn it off selectively when needed 🙏

[ToruNiina]

Yesterday I triggered CI for the branch that applied these changes to the latest main branch(https://github.com/ToruNiina/toml11/tree/rerun-295). And it passed. The changes itself also looks good to me, so I'll merge it. Sorry for the delay!

[ax3l]

Thank you @ToruNiina!

We are trying to transition a few more projects to C++20 and this patch would help to deal with older clang compilers (like v13.0).

Do you think you could include this in a 4.5.0 release of toml11? 🙏

[ax3l]

Hi @ToruNiina,

Can you potentially draft a new release for TOML11 with fixes like those? 🙏