Tests: Add FREECAD_USE_EXTERNAL_GTEST cmake option (#21190)

Add FREECAD_USE_EXTERNAL_GTEST cmake option and fix build with external libraries

cMake/FreeCAD_Helpers/InitializeFreeCADBuildOptions.cmake

@@ -11,6 +11,7 @@ macro(InitializeFreeCADBuildOptions)
     option(FREECAD_USE_EXTERNAL_FMT "Use system installed fmt library if available instead of fetching the source." ON)
     option(FREECAD_USE_EXTERNAL_ONDSELSOLVER "Use system installed OndselSolver instead of git submodule." OFF)
     option(FREECAD_USE_EXTERNAL_E57FORMAT "Use system installed libE57Format instead of the bundled." OFF)
+    option(FREECAD_USE_EXTERNAL_GTEST "Use system installed Google Test and Google Mock" OFF)
     option(FREECAD_USE_FREETYPE "Builds the features using FreeType libs" ON)
     option(FREECAD_BUILD_DEBIAN "Prepare for a build of a Debian package" OFF)
     option(FREECAD_CHECK_PIVY "Check for pivy version using Python at build time" ON)

tests/CMakeLists.txt

@@ -1,10 +1,10 @@
+if(FREECAD_USE_EXTERNAL_GTEST)
+    find_package(GTest REQUIRED)
+    set(Google_Tests_LIBS ${GTEST_LIBRARIES} ${GMOCK_LIBRARIES})
+else()
     if ( EXISTS "${CMAKE_SOURCE_DIR}/tests/lib/googletest" )
         include_directories( ${CMAKE_SOURCE_DIR}/tests/lib/googletest/include/ )
         include_directories( ${CMAKE_SOURCE_DIR}/tests/lib/googlemock/include/ )
-else()
-    find_package(GTest)
-    if( GTest_FOUND )
-        message( STATUS "Found Google Test: version ${GTest_VERSION}" )
     else()
         message( SEND_ERROR "The Google Test submodule is not available. Please run git submodule update --init" )
     endif()
@@ -124,7 +124,7 @@ foreach (exe ${TestExecutables})
     add_executable(${exe})
 endforeach()
 
-if ( EXISTS "${CMAKE_SOURCE_DIR}/tests/lib/googletest" )
+if ( NOT FREECAD_USE_EXTERNAL_GTEST )
     add_subdirectory(lib)
 endif()
 add_subdirectory(src)