Suppress __STDC_VERSION__ warns for C++
MODIFY
include/mruby.h
include/mruby.h
#define mrb_assert_int_fit(t1,n,t2,max) ((void)0)
#endif
-#if __STDC_VERSION__ >= 201112L
+#if defined __STDC_VERSION__ && __STDC_VERSION__ >= 201112L
#define mrb_static_assert(exp, str) _Static_assert(exp, str)
#else
#define mrb_static_assert(exp, str) mrb_assert(exp)

MODIFY
include/mruby/common.h
include/mruby/common.h
MRB_BEGIN_DECL
/** Declare a function that never returns. */
-#if __STDC_VERSION__ >= 201112L
+#if defined __STDC_VERSION__ && __STDC_VERSION__ >= 201112L
# define mrb_noreturn _Noreturn
#elif defined __GNUC__ && !defined __STRICT_ANSI__
# define mrb_noreturn __attribute__((noreturn))

Suppress __STDC_VERSION__ warns for C++