xcode update broke C++ compilation

[totaam]

libvpx fails to build with no member named 'nullptr_t' in the global namespace.
Links:

• Apple forums: error: no member named 'nullptr_t' in the global namespace
• SO: Updating to macOS 13.3 broke compiling cpp

Something in our environment causes clang to fall over itself...
The problem is that the environment is mostly setup by jhbuild / meson.

[totaam]

$ xcode-select  -p
/Applications/Xcode.app/Contents/Developer

Assuming that the command line tools are installed, we may need to run as root:

xcode-select -s /Library/Developer/CommandLineTools

[totaam]

The failing build command: jhbuild buildone libvpx