Support external clang source directory in //tools/clang/CMakeList.txt

Chromium clang plugins depends on clang headers, and the configuration
file assumes it's checked out in a fixed path in the LLVM source tree.
That is inconvenient to build it out of tree.

This CL adjusts the include path setting for external checkout of clang.

Review-Url: https://codereview.chromium.org/2394203003
Cr-Commit-Position: refs/heads/master@{#423809}

tools/clang/CMakeLists.txt

@@ -19,15 +19,21 @@ else(UNIX)
 endif()
 
 include_directories("${CMAKE_SOURCE_DIR}/include"
-                    "${CMAKE_SOURCE_DIR}/tools/clang/include"
                     "${CMAKE_BINARY_DIR}/include"
                     "${CMAKE_BINARY_DIR}/tools/clang/include")
 
 link_directories("${CMAKE_SOURCE_DIR}/lib"
-                 "${CMAKE_SOURCE_DIR}/tools/clang/lib"
                  "${CMAKE_BINARY_DIR}/lib"
                  "${CMAKE_BINARY_DIR}/tools/clang/lib")
 
+if (DEFINED LLVM_EXTERNAL_CLANG_SOURCE_DIR)
+  include_directories("${LLVM_EXTERNAL_CLANG_SOURCE_DIR}/include")
+  link_directories("${LLVM_EXTERNAL_CLANG_SOURCE_DIR}/lib")
+else ()
+  include_directories("${CMAKE_SOURCE_DIR}/tools/clang/include")
+  link_directories("${CMAKE_SOURCE_DIR}/tools/clang/lib")
+endif ()
+
 # Tests for all enabled tools can be run by building this target.
 add_custom_target(cr-check-all COMMAND ${CMAKE_CTEST_COMMAND} -V)