Add explicit `CMAKE_DEBUG_POSTFIX` option

Enable generating different library name to be compatible with CMake's `FindGTest`.

googletest/CMakeLists.txt

@@ -27,6 +27,8 @@ option(
   "Build gtest with internal symbols hidden in shared libraries."
   OFF)
 
+set(CMAKE_DEBUG_POSTFIX "d" CACHE STRING "Generate debug library name with a postfix.")
+
 # Defines pre_project_set_up_hermetic_build() and set_up_hermetic_build().
 include(cmake/hermetic_build.cmake OPTIONAL)