CMake: allow googletest and sanitizers-cmake from parent projects

This makes it easier to embed cubeb within larger projects that already
provide googletest and/or sanitizers-cmake, by testing directly for what
is used rather than indirectly for .git folders.

CMakeLists.txt

@@ -19,17 +19,22 @@ set(CMAKE_C_STANDARD 99)
 set(CMAKE_CXX_STANDARD 11)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
 
-if(NOT EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/googletest/.git" OR
-   NOT EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/cmake/sanitizers-cmake/.git")
-  message(FATAL_ERROR "submodules not initialized: run\n\tgit submodule update --init --recursive\nin base git checkout")
+if(NOT COMMAND add_sanitizers)
+  list(APPEND CMAKE_MODULE_PATH "${PROJECT_SOURCE_DIR}/cmake/sanitizers-cmake/cmake")
+  find_package(Sanitizers)
+  if(NOT COMMAND add_sanitizers)
+    message(FATAL_ERROR "Could not find sanitizers-cmake: run\n\tgit submodule update --init --recursive\nin base git checkout")
+  endif()
 endif()
 
-list(APPEND CMAKE_MODULE_PATH "${CMAKE_SOURCE_DIR}/cmake/sanitizers-cmake/cmake")
-find_package(Sanitizers)
-
-add_definitions(-DGTEST_HAS_TR1_TUPLE=0)
-set(gtest_force_shared_crt ON CACHE BOOL "")
-add_subdirectory(googletest)
+if(NOT TARGET gtest_main)
+  if(NOT EXISTS "${PROJECT_SOURCE_DIR}/googletest/CMakeLists.txt")
+    message(FATAL_ERROR "Could not find googletest: run\n\tgit submodule update --init --recursive\nin base git checkout")
+  endif()
+  add_definitions(-DGTEST_HAS_TR1_TUPLE=0)
+  set(gtest_force_shared_crt ON CACHE BOOL "")
+  add_subdirectory(googletest)
+endif()
 
 set(CMAKE_C_VISIBILITY_PRESET hidden)
 set(CMAKE_CXX_VISIBILITY_PRESET hidden)