Enforce that libc++ and libc++abi are built in a monorepo layout

[ldionne]

Summary

We will soon start removing technical debt and sharing code between the
two directories, so this first step is meant to discover potential places
where the libraries are built outside of a monorepo layout. I imagine
this could happen as a remnant of the pre-monorepo setup.

This was discussed on the libcxx-dev mailing list and we got overall
consensus on the direction.

[phosek]

LGTM

[mgorny]

libcxx/CMakeLists.txt

4

How does this interface with using other C++ ABI libraries?

[ldionne]

libcxx/CMakeLists.txt

4

It makes no difference -- the requirement is only that the libcxxabi sources be available. Essentially, we want to start taking advantage of the fact that we have the monorepo to stop duplicating code.

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 3.4.3)
 
 if(POLICY CMP0042)
   cmake_policy(SET CMP0042 NEW) # Set MACOSX_RPATH=YES by default
 endif()

libcxxabi/CMakeLists.txt

 # See www/CMake.html for instructions on how to build libcxxabi with CMake.
 
+if (NOT IS_DIRECTORY "${CMAKE_CURRENT_LIST_DIR}/../libcxx")
+  message(FATAL_ERROR "libc++abi now requires being built in a monorepo layout with libcxx available")
+endif()
+
 #===============================================================================
 # Setup Project
 #===============================================================================
 
 cmake_minimum_required(VERSION 3.4.3)
 
 if(POLICY CMP0042)
   cmake_policy(SET CMP0042 NEW) # Set MACOSX_RPATH=YES by default