Support old C++ compilers that don’t have <filesystem>

bpftrace uses <filesystem> which is relatively new in the C++ world. Amazon Linux 2 has g++ 7.x, which doesn’t have <filesystem> but has <experimental/filesystem> instead.

I initially used __cpp_lib_filesystem, but it is defined inside <filesystem>. So checking that to conditionally include <filesystem> doesn’t work.

Then there is <version> which defines __cpp_lib_filesystem and all other feature macros. But this header is also relatively new. Loading this header itself may need another #ifdef.

I ended up having __has_include (<filesystem>). Apparently it is in GNU cpp and Clang, but C++ 17 makes it “standard”. Yay!