Disable std::filesystem on macOS when compiling C++17 but targetting <10.15

[jagerman]

macOS flat out refuses to compile if you touch std::filesystem::path when targetting macOS < 10.15 (to be able to deploy a binary to older macOS versions) with the -mmacosx-version-min=10.14 (or earlier) compiler flags.

If you try to compile code with ::path (even if you don't actually call it) the Apple clang gods smite you with:

foo.cpp:8:26: error: 'path' is unavailable: introduced in macOS 10.15
        std::filesystem::path p;
                         ^
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX11.3.sdk/usr/include/c++/v1/filesystem:738:24: note: 'path' has been explicitly marked unavailable here
class _LIBCPP_TYPE_VIS path {
                       ^

This is reportedly caused by Apple's C++ team being somewhat incompetent at actually updating libc++ when they push out new releases, so libc++ stl features (in this case, to get at the std::filesystem exceptions from libc++) often lag behind the compiler by years, and Apple never releases libc++ updates for older mac versions because they don't care because C++ isn't Swift and only Swift is great and good.

</rant>

Anyway, this PR works around the issue by disabling the std::filesystem::path support when in C++17 mode when targetting macOS before 10.15.

I also broke up the hairy macro conditions because it was already hard to follow and would have gotten worse if kept in a single condition.

[paulocoutinhox]

Please, can you check and merge?

[SRombauts]

Yes, nice, thank you!