Libc++abi: find libc++ headers w/o find_path

[eugenis]

Summary

With http://reviews.llvm.org/D11740, libc++ headers can not be used from the libc++ source directly, because they include an auto-generated header with ABI version numbers.

This change makes libc++abi pick up the headers in the binary dir. Because of cmake inclusion order, those headers are not ready yet when libc++abi CMakeLists is executed, and can not be found with find_path. Hardcode the path to the headers instead.

CMakeLists.txt

 option(LIBCXXABI_ENABLE_ASSERTIONS "Enable assertions independent of build mode." ON)
 option(LIBCXXABI_ENABLE_PEDANTIC "Compile with pedantic enabled." ON)
 option(LIBCXXABI_ENABLE_WERROR "Fail and stop if a warning is triggered." OFF)
 option(LIBCXXABI_USE_LLVM_UNWINDER "Build and use the LLVM unwinder." OFF)
 option(LIBCXXABI_ENABLE_THREADS "Build with threads enabled" ON)
 set(LIBCXXABI_GCC_TOOLCHAIN "" CACHE STRING "GCC toolchain for cross compiling.")
 set(LIBCXXABI_SYSROOT "" CACHE STRING "Sysroot for cross compiling.")
 set(LIBCXXABI_LIBCXX_LIBRARY_PATH "" CACHE STRING "The path to libc++ library.")
+set(LIBCXX_ABI_MAJOR_VERSION 1 CACHE STRING "Major ABI version of libc++.")
 
 # Default to building a shared library so that the default options still test
 # the libc++abi that is being built. There are two problems with testing a
 # static libc++abi. In the case of a standalone build, the tests will link the
 # system's libc++, which might not have been built against our libc++abi. In the
 # case of an in tree build, libc++ will prefer a dynamic libc++abi from the
 # system over a static libc++abi from the output directory.
 option(LIBCXXABI_ENABLE_SHARED "Build libc++abi as a shared library." ON)
 option(LIBCXXABI_ENABLE_STATIC "Build libc++abi as a static library." ON)
 
 if (NOT LIBCXXABI_ENABLE_SHARED AND NOT LIBCXXABI_ENABLE_STATIC)
   message(FATAL_ERROR "libc++abi must be built as either a shared or static library.")
 endif()
 
+if (NOT "${LLVM_MAIN_SRC_DIR}" STREQUAL "")
+  # When building as part of LLVM, libc++ headers will end up under
+  # ${CMAKE_BINARY_DIR}. They are not there yet, so we can not use find_path.
+  set(LIBCXXABI_LIBCXX_INCLUDES
+    ${CMAKE_BINARY_DIR}/include/c++/v${LIBCXX_ABI_MAJOR_VERSION})
+else()
   find_path(
     LIBCXXABI_LIBCXX_INCLUDES
     vector
     PATHS ${LIBCXXABI_LIBCXX_INCLUDES}
-        ${LIBCXXABI_LIBCXX_PATH}/include
+          ${CMAKE_BINARY_DIR}/include/c++/v${LIBCXX_ABI_MAJOR_VERSION}
           ${CMAKE_BINARY_DIR}/${LIBCXXABI_LIBCXX_INCLUDES}
-        ${LLVM_MAIN_SRC_DIR}/projects/libcxx/include
-        ${LLVM_INCLUDE_DIR}/c++/v1
+          ${LLVM_INCLUDE_DIR}/c++/v${LIBCXX_ABI_MAJOR_VERSION}
     )
+endif()
 
 set(LIBCXXABI_LIBCXX_INCLUDES "${LIBCXXABI_LIBCXX_INCLUDES}" CACHE PATH
     "Specify path to libc++ includes." FORCE)
 
 find_path(
   LIBCXXABI_LIBCXX_PATH
   test/libcxx/__init__.py
   PATHS ${LIBCXXABI_LIBCXX_PATH}