Simplify MSVC detection to `mrb_static_assert`
#endif
#if (defined __cplusplus && __cplusplus >= 201103L) || \
- (defined _WIN32 && !defined __MINGW32__) || \
+ (defined _MSC_VER) || \
(defined __GXX_EXPERIMENTAL_CXX0X__) /* for old G++/Clang++ */
# define mrb_static_assert(exp, str) static_assert(exp, str)
#elif defined __STDC_VERSION__ && \
