Silence "expression result unused" warnings in AssertVariableIsOfTypeMacro

At least clang 3.1 generates those warnings. Prepend (void) to avoid them,
like we have in AssertMacro.

src/include/c.h

@@ -736,15 +736,15 @@ typedef NameData *Name;
 	StaticAssertStmt(__builtin_types_compatible_p(__typeof__(varname), typename), \
 	CppAsString(varname) " does not have type " CppAsString(typename))
 #define AssertVariableIsOfTypeMacro(varname, typename) \
-	StaticAssertExpr(__builtin_types_compatible_p(__typeof__(varname), typename), \
-	CppAsString(varname) " does not have type " CppAsString(typename))
+	((void) StaticAssertExpr(__builtin_types_compatible_p(__typeof__(varname), typename), \
+	 CppAsString(varname) " does not have type " CppAsString(typename)))
 #else /* !HAVE__BUILTIN_TYPES_COMPATIBLE_P */
 #define AssertVariableIsOfType(varname, typename) \
 	StaticAssertStmt(sizeof(varname) == sizeof(typename), \
 	CppAsString(varname) " does not have type " CppAsString(typename))
 #define AssertVariableIsOfTypeMacro(varname, typename) \
-	StaticAssertExpr(sizeof(varname) == sizeof(typename), \
-	CppAsString(varname) " does not have type " CppAsString(typename))
+	((void) StaticAssertExpr(sizeof(varname) == sizeof(typename),		\
+	 CppAsString(varname) " does not have type " CppAsString(typename)))
 #endif /* HAVE__BUILTIN_TYPES_COMPATIBLE_P */