utils: Add some initial build time assertion macros

These are useful to assert constants during build time. We evaluate the
expression to 0 when valid, so we can safely use the evaluated value.

src/libstrongswan/utils/utils.h

@@ -59,6 +59,20 @@
  */
 #define BUF_LEN 512
 
+/**
+ * Build assertion macro for integer expressions, evaluates to 0
+ */
+#define BUILD_ASSERT(x) (sizeof(char[(x) ? 0 : -1]))
+
+/**
+ * Build time check to assert a is an array, evaluates to 0
+ *
+ * The address of an array element has a pointer type, which is not compatible
+ * to the array type.
+ */
+#define BUILD_ASSERT_ARRAY(a) \
+		BUILD_ASSERT(!__builtin_types_compatible_p(typeof(a), typeof(&(a)[0])))
+
 /**
  * General purpose boolean type.
  */