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

src/cc65/compile.c

@@ -61,6 +61,7 @@
 #include "pragma.h"
 #include "preproc.h"
 #include "standard.h"
+#include "staticassert.h"
 #include "symtab.h"
 
 
@@ -108,6 +109,12 @@ static void Parse (void)
             continue;
         }
 
+        /* Check for a _Static_assert */
+        if (CurTok.Tok == TOK_STATIC_ASSERT) {
+            ParseStaticAssert ();
+            continue;
+        }
+
         /* Read variable defs and functions */
         ParseDeclSpec (&Spec, SC_EXTERN | SC_STATIC, T_INT);