CMake: use `TINT_THIRD_PARTY_DIR`

Instead of '${TINT_ROOT_SOURCE_DIR}/third_party'

Fixed: tint:1435
Change-Id: I80622f490326ccba4580021dfe2b355a7586e307
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/80760
Auto-Submit: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>
diff --git a/CMakeLists.txt b/CMakeLists.txt
index c26ae7e..f929ca9 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -198,7 +198,7 @@
 endif()
 
 if (${TINT_BUILD_SPV_READER})
-  include_directories("${TINT_ROOT_SOURCE_DIR}/third_party/spirv-tools/include")
+  include_directories("${TINT_THIRD_PARTY_DIR}/spirv-tools/include")
 endif()
 
 if((CMAKE_CXX_COMPILER_ID STREQUAL "Clang") AND (CMAKE_CXX_SIMULATE_ID STREQUAL "MSVC"))
@@ -262,7 +262,7 @@
 
   if (${TINT_BUILD_SPV_READER} OR ${TINT_BUILD_SPV_WRITER})
     target_include_directories(${TARGET} PUBLIC
-        "${TINT_ROOT_SOURCE_DIR}/third_party/spirv-headers/include")
+        "${TINT_THIRD_PARTY_DIR}/spirv-headers/include")
   endif()
 
   target_compile_definitions(${TARGET} PUBLIC -DTINT_BUILD_SPV_READER=$<BOOL:${TINT_BUILD_SPV_READER}>)