provide std::filesystem library for Raspbian stable clang

[mcandre]

Either:

• [ ] Replace RaspbiOS 10 with an alternative operating system featuring a more recent clang v9+

Or:

• [ ] Upgrade RPi to a testing edition of Bullseye

Or:

• [ ] Upgrade clang to at least v9

Or:

• [ ] Install a clang 7 compatible std::filesystem implementation

Or:

• [ ] Detect when the environment is Description: Raspbian GNU/Linux 10 in lsb_release -a / when CLANG_VERSION_MAJOR is less than 9
• [ ] Apply -lstdc++fs (gcc), or possibly -lc++fs (clang).

[mcandre]

Release media works for launching rez, but the user rez.cpp file still won't be able to access std::filesystem without further toolchain work.