build: make NM_MORE_ASSERTS define numeric for different levels of more-asserts

Allows to enable more-asserts more granularly.

Unfortunately, the old check was "${enable_more_asserts} == "yes", thus
we cannot extend "--enable-more-assert=level" because that would mean
that the same build script cannot set the option on both old and new
NetworkManager.
Thus, add a new option --with-more-asserts=level. If you put the
following in your build script, it will work as expected whether
you build a new or an old version of NetworkManager.
  ./configure --enable-more-asserts --with-more-asserts=5

src/nm-bus-manager.c

@@ -106,7 +106,7 @@ nm_bus_manager_setup (NMBusManager *instance)
 static void
 nm_assert_exported (NMBusManager *self, const char *path, NMExportedObject *object)
 {
-#ifdef NM_MORE_ASSERTS
+#if NM_MORE_ASSERTS
 	NMBusManagerPrivate *priv;
 	const char *p2, *po;
 	NMExportedObject *o2;