build(gtest): added gtest as a subproject in case it is not already installed on the system

subprojects/googletest-1.15.2/CMakeLists.txt

@@ -0,0 +1,36 @@
+# Note: CMake support is community-based. The maintainers do not use CMake
+# internally.
+
+cmake_minimum_required(VERSION 3.13)
+
+project(googletest-distribution)
+set(GOOGLETEST_VERSION 1.15.2)
+
+if(NOT CYGWIN AND NOT MSYS AND NOT ${CMAKE_SYSTEM_NAME} STREQUAL QNX)
+  set(CMAKE_CXX_EXTENSIONS OFF)
+endif()
+
+enable_testing()
+
+include(CMakeDependentOption)
+include(GNUInstallDirs)
+
+# Note that googlemock target already builds googletest.
+option(BUILD_GMOCK "Builds the googlemock subproject" ON)
+option(INSTALL_GTEST "Enable installation of googletest. (Projects embedding googletest may want to turn this OFF.)" ON)
+option(GTEST_HAS_ABSL "Use Abseil and RE2. Requires Abseil and RE2 to be separately added to the build." OFF)
+
+if(GTEST_HAS_ABSL)
+  if(NOT TARGET absl::base)
+    find_package(absl REQUIRED)
+  endif()
+  if(NOT TARGET re2::re2)
+    find_package(re2 REQUIRED)
+  endif()
+endif()
+
+if(BUILD_GMOCK)
+  add_subdirectory( googlemock )
+else()
+  add_subdirectory( googletest )
+endif()