[libc++] Do not set CMAKE_BUILD_WITH_INSTALL_NAME_DIR

[ldionne]

Summary

I'm not sure what that gains us, and it creates a problem when
trying to run the tests against libc++ with a custom install name
dir (e.g. /usr/lib), since the library that we link against (in
the build tree) will advertise itself as /usr/lib/libc++.dylib,
so we end up linking against the system dylib at runtime.

libcxx/CMakeLists.txt

 # See https://libcxx.llvm.org/docs/BuildingLibcxx.html for instructions on how
 # to build libcxx with CMake.
 
 if (NOT IS_DIRECTORY "${CMAKE_CURRENT_LIST_DIR}/../libcxxabi")
   message(FATAL_ERROR "libc++ now requires being built in a monorepo layout with libcxxabi available")
 endif()
 
 #===============================================================================
 # Setup Project
 #===============================================================================
 cmake_minimum_required(VERSION 3.13.4)
 
-set(CMAKE_BUILD_WITH_INSTALL_NAME_DIR ON)
-
 # Add path for custom modules
 set(CMAKE_MODULE_PATH
   "${CMAKE_CURRENT_SOURCE_DIR}/cmake"
   "${CMAKE_CURRENT_SOURCE_DIR}/cmake/Modules"
   ${CMAKE_MODULE_PATH}
   )
 
 set(CMAKE_FOLDER "libc++")

libcxx/utils/ci/runtimes/CMakeLists.txt

 cmake_minimum_required(VERSION 3.13.4)
 project(LLVM_RUNTIMES C CXX ASM)
 
-set(CMAKE_BUILD_WITH_INSTALL_NAME_DIR ON)
-
 find_package(Python3 COMPONENTS Interpreter)
 if(NOT Python3_Interpreter_FOUND)
   message(WARNING "Python3 not found, using python2 as a fallback")
   find_package(Python2 COMPONENTS Interpreter REQUIRED)
   if(Python2_VERSION VERSION_LESS 2.7)
     message(SEND_ERROR "Python 2.7 or newer is required")
   endif()