Add a macro for C++20 constexpr

For now, there is only a single use of this macro. So there is no need to add it now. There may be more uses once we begin making more use of C++20. 

Some changes and enhancements were made to the constexpr feature in C++20 making it more flexible.  For example: It allows now dynamic memory allocation and deallocation, constexpr destructors, ect.  https://en.cppreference.com/w/cpp/language/constexpr#:~:text=Constexpr%20lambda-,201907L,-(C%2B%2B20)}}

Create a macro for C++20 constexpr instead of writing multiple times:

#if __cpp_constexpr >= 201907L

    constexpr

#endif 

I suggest a simple macro CONSTEXPR_20 in qcompilerdetection.h:

#if __cpp_constexpr >= 201907L

    define CONSTEXPR_20     constexpr
#else

   define CONSTEXPR_20
#endif