Fix CMakeLists.txt and include directive

[NobilisNC]

I was trying to make a vcpkg port of the restless library.
To achieve this I need to port dependencies, including this library but there are some issues with the CMakeLists.txt and a missing include.

In CMakeLists.txt :

• Add an option to build tests
• Add /Za compiler flag to enable the use of operators not_eq, or, and, etc...

In settings_impl.hpp :

• Add <cctype> to use std::tolower