Fix build warnings for Clang and MSVC

[WimLeflere]

Fix some build warnings for Clang and MSVC.

• unnecessary semicolons
• missing virtual desctructor
• implicit conversion from double to int

At the moment I have to disable some warnings when inja is included.
It would be nice if this is no longer needed.

#ifdef _MSC_VER
#pragma warning(push)
#pragma warning(disable : 4244)
#else
#pragma clang diagnostic push
#pragma clang diagnostic warning "-Wextra-semi"
#pragma clang diagnostic warning "-Wnon-virtual-dtor"
#endif
#include <inja/inja.hpp>
#ifdef _MSC_VER
#pragma warning(pop)
#else
#pragma clang diagnostic pop
#endif

[pantor]

Thanks so much!