Codechange: use "[[maybe_unused]]" instead of a wide variety of other ways we had

While at it, replace OTTD_ASSERT with WITH_ASSERT, as this
is always set if assert() is valid. No matter if NDEBUG is set
or not.

src/core/pool_func.hpp

@@ -33,9 +33,9 @@ DEFINE_POOL_METHOD(inline)::Pool(const char *name) :
 		first_free(0),
 		first_unused(0),
 		items(0),
-#ifdef OTTD_ASSERT
+#ifdef WITH_ASSERT
 		checked(0),
-#endif /* OTTD_ASSERT */
+#endif /* WITH_ASSERT */
 		cleaning(false),
 		data(nullptr),
 		alloc_cache(nullptr)
@@ -133,10 +133,10 @@ DEFINE_POOL_METHOD(void *)::GetNew(size_t size)
 {
 	size_t index = this->FindFirstFree();
 
-#ifdef OTTD_ASSERT
+#ifdef WITH_ASSERT
 	assert(this->checked != 0);
 	this->checked--;
-#endif /* OTTD_ASSERT */
+#endif /* WITH_ASSERT */
 	if (index == NO_FREE_ITEM) {
 		error("%s: no more free items", this->name);
 	}