BUILD.md: Add note about 'span' not found (#646)

Author: dougthor42

BUILD.md

@@ -59,10 +59,11 @@ You can use GCC by doing the following:
  - If step 3 fails while compiling the boost-build dependency, it means you don't have a working/good standard library installation. Check the integrity of your system headers and making sure that C++ related packages are installed and intact.
  - If step 3 gives a random error, read the `[package-name-and-platform]-out.log` and `[package-name-and-platform]-err.log` for the actual reason to see if you might be lacking the headers from a dependency.
  - If step 3 is still failing or if you're not able to find the cause, please make an issue on our Github about it!
+ - If step 3 fails during rebuild after `git pull` with an error that mentions RPATH, add this to the end of step 3: `-DCMAKE_BUILD_WITH_INSTALL_RPATH=ON`
  - If step 4 gives you an error that contains something like `main.cpp.o: in function 'std::__cxx11::basic_string...`, you likely are experiencing a clang-14 issue. This can only be fixed by either lowering the clang version or using GCC, see below.
  - If step 4 gives you a different error, you could report it to this repo or try using GCC. Just make sure your standard library and compilers are updated since Cemu uses a lot of modern features!
  - If step 4 gives you undefined libdecor_xx, you are likely experiencing an issue with sdl2 package that comes with vcpkg. Delete sdl2 from vcpkg.json in source file and recompile.
- - If step 3 fails during rebuild after `git pull` with an error that mentions RPATH, add this to the end of step 3: `-DCMAKE_BUILD_WITH_INSTALL_RPATH=ON`
+ - If step 4 gives you `fatal error: 'span' file not found`, then you're either missing `libstdc++` or are using a version that's too old. Install at least v10 with your package manager, eg `sudo apt install libstdc++-10-dev`. See #644.
 
 ## macOS