Cmake build fails if using Xcode generator

[frenchdog]

(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".