Update README instructions for building on macOS

The build commands in the README required some tweaking for me to build the project on macOS. It might just be my machine, but if this is a recurring issue here are the commands that worked for me (with the latest Xcode 14.0 beta installed). It might be worthwhile adding this to the README (I'm happy to PR if other mac users can reproduce the same issues).

# ... (the steps proceeding configuring and building with cmake didn't need tweaking)

cmake -D CMAKE_C_COMPILER="$(xcrun -find cc)" -D CMAKE_CXX_COMPILER="$(xcrun -find c++)" -D CMAKE_CXX_FLAGS="${CMAKE_CXX_FLAGS} -std=c++17 -stdlib=libc++ -std=c++1z" -G Xcode ..
cmake --build . --config Release

Issues this fixed

Without CMAKE_C_COMPILER and CMAKE_CXX_COMPILER set to those values, I get the error briefly mentioned in the README. Without the CMAKE_CXX_FLAGS that set the standard to c++17 I run into a lot of syntax errors (it seems to compile with something from before c++11 by default). It should be noted that the CMAKE_CXX_STANDARD option didn't seem to change anything at all so I had to use CMAKE_CXX_FLAGS instead.

I hardly ever touch cpp, so there might be something weird about those commands, but at least they work. Feel free to suggest changes.

Review the existing macOS build instructions in the README and compare them with the reported CMake commands using Xcode 14. Verify which compiler, C++ standard, and generator settings are required on macOS. Done means the README contains accurate reproducible instructions for building the project on macOS.