Fix libcxxabi CMake detection of libcxx path to handle LLVM_EXTERNAL_LIBCXX_SOURCE_DIR flag

libcxxabi/trunk/CMakeLists.txt

 # 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 (LLVM_EXTERNAL_LIBCXX_SOURCE_DIR)
+  set(LIBCXXABI_LIBCXX_SRC_DIR ${LLVM_EXTERNAL_LIBCXX_SOURCE_DIR})
+else()
+  set(LIBCXXABI_LIBCXX_SRC_DIR ${LLVM_MAIN_SRC_DIR}/projects/libcxx/include)
+endif()
+
 find_path(
   LIBCXXABI_LIBCXX_INCLUDES
   vector
   PATHS ${LIBCXXABI_LIBCXX_INCLUDES}
         ${LIBCXXABI_LIBCXX_PATH}/include
         ${CMAKE_BINARY_DIR}/${LIBCXXABI_LIBCXX_INCLUDES}
-        ${LLVM_MAIN_SRC_DIR}/projects/libcxx/include
+        ${LIBCXXABI_LIBCXX_SRC_DIR}/include
         ${LLVM_INCLUDE_DIR}/c++/v1
   )
 
 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}
         ${LIBCXXABI_LIBCXX_INCLUDES}/../
-        ${LLVM_MAIN_SRC_DIR}/projects/libcxx/
+        ${LIBCXXABI_LIBCXX_SRC_DIR}
   NO_DEFAULT_PATH
   )
 
 if (LIBCXXABI_LIBCXX_PATH STREQUAL "LIBCXXABI_LIBCXX_PATH-NOTFOUND")
   message(WARNING "LIBCXXABI_LIBCXX_PATH was not specified and couldn't be infered.")
   set(LIBCXXABI_LIBCXX_PATH "")
 endif()