Recommend do not override CMAKE_INSTALL_PREFIX on MSVC

I'm a Windows user, I don't actually want this--I can set CMAKE_INSTALL_PREFIX myself if necessary.  When I install websocketpp, I intend for the installation to be to a place that's similar in sense to what it is on Unix systems, which on Windows is generally Program Files.  By allowing the CMAKE_INSTALL_PREFIX path to simply default, not only does this default behavior automatically appear, but it also corrects a problem currently existing with the INSTALL project which causes the websocketpp-config.cmake installed by the INSTALL project to contain invalid entries.
CMake configure_file should place config.cmake in the root

This change allows the configuration of websocketpp-config.cmake to be compliant with what cmake expects when an "export" statement is used.  This adjustment allows the "find_package(websocketpp)" command to behave as expected if a user installs websocketpp via the "INSTALL" project on Windows.
Merge commit 'ba2f4227198c7480994cea07f54675b1c4531baa' into ref-websocketpp-subtree