Use cmake cache variables to customize build settings

[hotwatermorning]

Hi.

I want to use IXWebSocket library with C++17 project but it seems that IXWebSocket can be built only for C++11.

Thus I modified CMakeLists.txt to use cache variables to enable customizing build settings outside of the file. (And fixed CXX_STANDARD_REQUIRED usage. It should be CMAKE_CXX_STANDARD_REQUIRED because CXX_STANDARD_REQUIRED is a property. https://cmake.org/cmake/help/latest/prop_tgt/CXX_STANDARD_REQUIRED.html )

Thanks.