[libcxxabi] Prevent cmake from removing our explicit system C++ include paths

We build with `-nostdinc++` and add our own header path via
`LIBCXXABI_LIBCXX_INCLUDES`.  However cmake tried to be clever and if
`LIBCXXABI_LIBCXX_INCLUDES` happens to match the compilers system path
it will remove the `-I` flag meaning we can't access any C++ headers.

Ideally cmake would be able see that we are using `-nostdinc++` and
disable this behaviour.

Differential Revision: https://reviews.llvm.org/D69973

diff --git a/libcxxabi/CMakeLists.txt b/libcxxabi/CMakeLists.txt
index fafb777..d914b6e 100644 (file)
--- a/libcxxabi/CMakeLists.txt
+++ b/libcxxabi/CMakeLists.txt
@@ -270,6 +270,13 @@ include(config-ix)
 
 if (LIBCXXABI_HAS_NOSTDINCXX_FLAG)
   list(APPEND LIBCXXABI_COMPILE_FLAGS -nostdinc++)
+  # cmake 3.14 and above remove system include paths that are explicitly
+  # passed on the command line.  We build with -nostdinc++ and explicitly add
+  # just the libcxx system include paths with -I on the command line.
+  # Setting CMAKE_CXX_IMPLICIT_INCLUDE_DIRECTORIES effectively prevents cmake
+  # from removing these.
+  # See: https://gitlab.kitware.com/cmake/cmake/issues/19227
+  set(CMAKE_CXX_IMPLICIT_INCLUDE_DIRECTORIES "")
   # Remove -stdlib flags to prevent them from causing an unused flag warning.
   string(REPLACE "-stdlib=libc++" "" CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}")
   string(REPLACE "-stdlib=libstdc++" "" CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}")