Cmake build fails if using Xcode generator

(Reported by @dgovil)

The CMake build fails if using the Xcode generator. Using the Unix Makefiles generator works fine, but maybe it's worth calling out in the build instructions to use the Makefile generator?
The error is

```
CMake Error in cmake/bifusd/src/config/CMakeLists.txt:
The custom command generating

/Users/dhruvgovil/Projects/bifrost-usd/apple/build_scripts/build/bifrost_usd_build_maya2023/cmake/bifusd/src/config/bifusd/config/CfgCompilerMacros.cpp

is attached to multiple targets:

cg-CfgCompilerMacros.h-0acc790d41c
hc-CfgCompilerMacros.h-0acc790d41c

but none of these is a common dependency of the other(s). This is not
allowed by the Xcode "new build system".
```

Start by reproducing the failure with the Xcode generator and compare it with the working Unix Makefiles generator. Inspect cmake/bifusd/src/config/CMakeLists.txt and determine whether the expected outcome is a build fix or a documented generator restriction; done means the chosen scope is explicit and verified.

Written by the indexing model from the issue text.