message: build: use -stdlib++-isystem with Clang 10
diff: @@ -60,16 +60,11 @@ $(foreach TOOL,$(cctools_TOOLS),$(eval darwin_$(TOOL) = $$(build_prefix)/bin/$$(
  # Explicitly point to our binaries (e.g. cctools) so that they are
  # ensured to be found and preferred over other possibilities.
  #
  -# -stdlib=libc++ -nostdinc++ -Xclang -cxx-isystem$(OSX_SDK)/usr/include/c++/v1
  +# -stdlib=libc++ -stdlib++-isystem$(OSX_SDK)/usr/include/c++/v1
  #
  # Forces clang to use the libc++ headers from our SDK and completely
  # forget about the libc++ headers from the standard directories
  #
  -# TODO: Once we start requiring a clang version that has the
  -# -stdlib++-isystem<directory> flag first introduced here:
  -# https://reviews.llvm.org/D64089, we should use that instead. Read the
  -# differential summary there for more details.
  -#
  # -Xclang -*system<path_a> \
  # -Xclang -*system<path_b> \
  # -Xclang -*system<path_c> ...
  @@ -109,8 +104,8 @@ darwin_CXX=env -u C_INCLUDE_PATH -u CPLUS_INCLUDE_PATH \
  $(clangxx_prog) --target=$(host) -mmacosx-version-min=$(OSX_MIN_VERSION) \
  -B$(build_prefix)/bin -mlinker-version=$(LD64_VERSION) \
  -isysroot$(OSX_SDK) \
  - -stdlib=libc++ -nostdinc++ \
  - -Xclang -cxx-isystem$(OSX_SDK)/usr/include/c++/v1 \
  + -stdlib=libc++ \
  + -stdlib++-isystem$(OSX_SDK)/usr/include/c++/v1 \
  -Xclang -internal-externc-isystem$(clang_resource_dir)/include \
  -Xclang -internal-externc-isystem$(OSX_SDK)/usr/include
  