Index: libcxxabi/trunk/CMakeLists.txt
===================================================================
--- libcxxabi/trunk/CMakeLists.txt
+++ libcxxabi/trunk/CMakeLists.txt
@@ -137,13 +137,19 @@
   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
   )
 
@@ -155,7 +161,7 @@
   test/libcxx/__init__.py
   PATHS ${LIBCXXABI_LIBCXX_PATH}
         ${LIBCXXABI_LIBCXX_INCLUDES}/../
-        ${LLVM_MAIN_SRC_DIR}/projects/libcxx/
+        ${LIBCXXABI_LIBCXX_SRC_DIR}
   NO_DEFAULT_PATH
   )