diff --git a/CMakeLists.txt b/CMakeLists.txt
index af88762..d8ad27d 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -92,6 +92,8 @@ if(libmahjong_use_clang_utils AND NOT CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
 
 endif()
 
+add_subdirectory(extern)
+
 add_library(libmahjong SHARED)
 add_subdirectory(src)
 
diff --git a/extern/CMakeLists.txt b/extern/CMakeLists.txt
new file mode 100644
index 0000000..85d088f
--- /dev/null
+++ b/extern/CMakeLists.txt
@@ -0,0 +1,13 @@
+include(FetchContent)
+set(CMAKE_CXX_CLANG_TIDY "")
+
+# Running -fsanitize=msan with clang requires we build gtest with it
+# instrumented.
+if(libmahjong_build_tests AND libmahjong_enable_msan)
+  FetchContent_Declare(
+    googletest
+    GIT_REPOSITORY https://github.com/google/googletest.git
+    GIT_TAG v1.17.0
+    GIT_SHALLOW)
+  FetchContent_MakeAvailable(googletest)
+endif()