cxx_strict_flags was omitted in several compiler conditional blocks
(such as MSVC, SunPro, XL, and HP), leading to a CMake developer
warning about an uninitialized variable when dev warnings are enabled.
Fixes#4071
The fallback check GTEST_INTERNAL_HAS_INCLUDE(<compare>) && C++20 causes build failures on Android NDK 24 which has the header but lacks the full comparison operators needed by PrintTo. The feature test macro __cpp_lib_three_way_comparison is the reliable indicator.
Fixes#4933