[cmake] [libcxx] Fix find_path() problems when cross compiling.

[hintonda]

Summary

When CMAKE_SYSROOT or CMAKE_FIND_ROOT_PATH is set, cmake
recommends setting CMAKE_FIND_ROOT_PATH_MODE_INCLUDE=ONLY
globally which means find_path() always prepends CMAKE_SYSROOT or
CMAKE_FIND_ROOT_PATH to all paths used in the search.

However, this find_path() invocation is looking for a path in the
libcxxabi project on the host system, not the target system,
which can be done by passing NO_CMAKE_FIND_ROOT_PATH.

[compnerd]

Similar to the libc++abi wrt find_path. The CMAKE_REQUIRED_FLAGS handling LGTM.

[phosek]

Would it be possible to split the CMAKE_REQUIRED_FLAGS fix into a separate change since it's unrelated to the CMAKE_FIND_ROOT_PATH bit?

[hintonda]

In D41622#965280, @phosek wrote:

Would it be possible to split the CMAKE_REQUIRED_FLAGS fix into a separate change since it's unrelated to the CMAKE_FIND_ROOT_PATH bit?

Sure, but both are needed to actually work. This fixes cross compiling across systems, e.g., from Darwin to Linux. Without both, configuration will fail.

Please note, cross system compiling also requires a pending patch to cmake: https://gitlab.kitware.com/cmake/cmake/merge_requests/1620

[hintonda]

Btw, here's the WIP toolchain file I"m using, which might help clear up why these changes is necessary:

https://github.com/donhinton/misc/blob/master/cmake/linux-toolchain.cmake

[hintonda]

ping...

[hintonda]

• Save CMAKE_REQUIRED_FLAGS for another patch.

libcxx/trunk/CMakeLists.txt

 if (LIBCXX_CXX_ABI STREQUAL "default")
   find_path(
     LIBCXX_LIBCXXABI_INCLUDES_INTERNAL
     cxxabi.h
     PATHS ${LLVM_MAIN_SRC_DIR}/projects/libcxxabi/include
           ${LLVM_MAIN_SRC_DIR}/runtimes/libcxxabi/include
           ${LLVM_MAIN_SRC_DIR}/../libcxxabi/include
     NO_DEFAULT_PATH
+    NO_CMAKE_FIND_ROOT_PATH
   )
   if (LIBCXX_TARGETING_MSVC)
     # FIXME: Figure out how to configure the ABI library on Windows.
     set(LIBCXX_CXX_ABI_LIBNAME "vcruntime")
   elseif ((NOT LIBCXX_STANDALONE_BUILD OR HAVE_LIBCXXABI) AND
           IS_DIRECTORY "${LIBCXX_LIBCXXABI_INCLUDES_INTERNAL}")
     set(LIBCXX_CXX_ABI_LIBNAME "libcxxabi")
     set(LIBCXX_CXX_ABI_INCLUDE_PATHS "${LIBCXX_LIBCXXABI_INCLUDES_INTERNAL}")