enable parallel compilation of unity target for MSVC

CMake/cotire.cmake

@@ -3309,6 +3309,13 @@ function (cotire_setup_unity_build_target _languages _configurations _target)
 			set_property(TARGET ${_unityTargetName} PROPERTY ENABLE_EXPORTS TRUE)
 		endif()
 	endif()
+	# enable parallel compilation for MSVC
+	if (MSVC AND "${CMAKE_GENERATOR}" MATCHES "Visual Studio")
+		list (LENGTH _unityTargetSources _numberOfUnityTargetSources)
+		if (_numberOfUnityTargetSources GREATER 1)
+			set_property(TARGET ${_unityTargetName} APPEND PROPERTY COMPILE_OPTIONS "/MP")
+		endif()
+	endif()
 	cotire_init_target(${_unityTargetName})
 	cotire_add_to_unity_all_target(${_unityTargetName})
 	set_property(TARGET ${_target} PROPERTY COTIRE_UNITY_TARGET_NAME "${_unityTargetName}")