Make SYSTEM argument optional in target_include_directories

[KOMMYHAP]

Hello! I'd like to use cmake FetchContent to download googletest sources and clang-cl to compile it. But in this case gtest include directory was not added to gtest target. Seems like SYSTEM argument in target_include_directories does not pollute target properties correctly. I wrap SYSTEM argument in cmake option gtest_enable_system_include to save backward compatibility.