Installation issues with Xcode 15

With Xcode 15, some functions have been removed from the standard library, specifically std::unary_function and std::binary_function. This is also the case for other libcpp versions (>= 15). This leads to compilation failing somewhere in boost.

One way to fix this is to re-enable them by setting _LIBCPP_ENABLE_CXX17_REMOVED_UNARY_BINARY_FUNCTION.

Supposedly, it should also be possible to tell boost to not use these functions by setting BOOST_NO_CXX98_FUNCTION_BASE. However, I did not get that to work ... and believe that for boost >= 1.81 this should be done automatically (but seems not to have an effect).