Fixed RasPi builds of unit tests.

On RasPi with gcc 4.8.2, the asserts wouldn't compile when tests were enabled.
Enforced the assumption that ASSERT code is generated only in Debug builds.

src/BlockID.cpp

@@ -392,7 +392,9 @@ AString DamageTypeToString(eDamageType a_DamageType)
 
 	// Unknown damage type:
 	ASSERT(!"Unknown DamageType");
-	return Printf("dtUnknown_%d", static_cast<int>(a_DamageType));
+	#ifndef __clang__
+		return Printf("dtUnknown_%d", static_cast<int>(a_DamageType));
+	#endif
 }