auto-detect the presence of the std::format & {fmt} libraries in the C++ code.

[GerHobbelt]

auto-detect the presence of the std::format & {fmt} libraries by checking for their header files (in platform). This automatically sets up the LIBASSERT_USE_FMT & LIBASSERT_USE_STD_FORMAT defines.

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At least this is useful to me as we don't use CMake; when the include paths are set up to point at those libraries, this should pick them up across the build.