fixed defines for NK_SIZE_TYPE and NK_POINTER_TYPE on MinGW and windows clang/clang-cl.

[m256i]

Issue as stated in #810
modified defines for NK_SIZE_TYPE and NK_POINTER_TYPE to:

    #elif defined(_WIN64) && (defined(__MINGW64__) || defined(__clang__))
      #define NK_SIZE/POINTER_TYPE unsigned long long
   ...
    #elif (defined(_WIN32) || defined(WIN32)) && (defined(__MINGW32__) || defined(__clang__))
      #define NK_SIZE/POINTER_TYPE unsigned long  

to account for MinGW 64 and 32 bit aswell as windows clang 64 and 32 bit and clang-cl 64 and 32 bit.
i've put together a quick goldbolt comparison
with MinGW gcc + clang and a gnu/linux gcc basecase, to demonstrate that the fix works as intended.

[m256i]

I have not tested building with clang-cl inside Vistual Studio.