Bugfix: fix compiler linker can't find std::filesystem

[JackeyLove1]

In some compilers that support only some C++17 features like gcc6.3.0, it may encounter a situation where std filesystem lib can't be found, like:

/usr/bin/ld: CMakeFiles/gemma.dir/gemma.cc.o: in function `std::filesystem::__cxx11::path::path<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::filesystem::__cxx11::path>(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, std::filesystem::__cxx11::path::format)':
[gemma.cc](http://gemma.cc/):(.text._ZNSt10filesystem7__cxx114pathC2INSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEES1_EERKT_NS1_6formatE[_ZNSt10filesystem7__cxx114pathC5INSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEES1_EERKT_NS1_6formatE]+0x74): undefined reference to `std::filesystem::__cxx11::path::_M_split_cmpts()'

So we need to explicitly link the std::filesystem library in cmake.

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[jan-wassenberg]

Thanks! Does this also work for clang, or do we require some logic to detect which one to link against? Something like https://github.com/ned14/llfio/issues/52 ?

[jan-wassenberg]

Thank you @friendlyanon , I agree this sounds like a good approach, let's go with that :) Please feel free to reopen if you'd like to discuss further.