Fix constexpr macros for MSVC STL

Author: void2012

nh3api/core/nh3api_std/nh3api_std.hpp

@@ -339,15 +339,15 @@
     #endif
 
     #ifndef NH3API_CONSTEXPR_CPP_20
-        #ifdef _CONSTEXPR20 // avoid stupid MSVC double inline warning
+        #if NH3API_CHECK_CPP20 && defined(_CONSTEXPR20)
             #define NH3API_CONSTEXPR_CPP_20 constexpr
         #else
             #define NH3API_CONSTEXPR_CPP_20
         #endif
     #endif
 
     #ifndef NH3API_CONSTEXPR_CPP_17
-        #ifdef _CONSTEXPR17 // avoid stupid MSVC double inline warning
+        #if NH3API_CHECK_CPP17 && defined(_CONSTEXPR17)
             #define NH3API_CONSTEXPR_CPP_17 constexpr
         #else
             #define NH3API_CONSTEXPR_CPP_17