Get include dirs from target rather than global variables.

The global variable gtest_SOURCE_DIR is replaced by CMake when fetch-content is used with the name "GTest" and points to the root directory. The include directories from the gtest target are always valid.

googlemock/CMakeLists.txt

@@ -65,12 +65,13 @@ endif()
 config_compiler_and_linker() # from ${gtest_dir}/cmake/internal_utils.cmake
 
 # Adds Google Mock's and Google Test's header directories to the search path.
+# Get Google Test's include dirs from the target, gtest_SOURCE_DIR is broken
+# when using fetch-content with the name "GTest".
+get_target_property(gtest_include_dirs gtest INCLUDE_DIRECTORIES)
 set(gmock_build_include_dirs
   "${gmock_SOURCE_DIR}/include"
   "${gmock_SOURCE_DIR}"
-  "${gtest_SOURCE_DIR}/include"
-  # This directory is needed to build directly from Google Test sources.
-  "${gtest_SOURCE_DIR}")
+  "${gtest_include_dirs}")
 include_directories(${gmock_build_include_dirs})
 
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