Add support for static_assert

Add C11's _Static_assert and static_assert macro.

This is like #error, but is handled at a later stage
of translation, so it is possible to check sizes of
types, values of enums, etc.

https://en.cppreference.com/w/c/language/_Static_assert
https://port70.net/~nsz/c/c11/n1570.html#6.7.10

include/assert.h

@@ -46,6 +46,12 @@ extern void __fastcall__ _afailed (const char*, unsigned);
 #  define assert(expr)  ((expr)? (void)0 : _afailed(__FILE__, __LINE__))
 #endif
 
+/*
+** TODO: Guard with #if __STDC_VERSION__ >= 201112L or similar when there
+** is a C11 mode.
+*/
+#define static_assert _Static_assert
+
 
 
 /* End of assert.h */