Check explicitly if __STDC_VERSION__ is defined

Author: vstinner

Include/pymacro.h

@@ -46,7 +46,7 @@
 /* Argument must be a char or an int in [-128, 127] or [0, 255]. */
 #define Py_CHARMASK(c) ((unsigned char)((c) & 0xff))
 
-#if __STDC_VERSION__ >= 201112L
+#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 201112L
 #  define Py_BUILD_ASSERT_EXPR(cond) \
     (sizeof(struct { _Static_assert(cond, #cond); int dummy; }), \
      0)
@@ -68,7 +68,7 @@
     (sizeof(char [1 - 2*!(cond)]) - 1)
 #endif
 
-#if __STDC_VERSION__ >= 201112L
+#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 201112L
    // Use static_assert() on C11 and newer
 #  define Py_BUILD_ASSERT(cond) static_assert((cond), #cond)
 #else