Makefiles: What is an order-only prerequisite?

First, let’s recall what the ordinary prerequisites are.

Basically, the prerequisites in Makefile have two functions:

1. They are checked and, if necessary, are built before the target
2. If any of the prerequisites gets rebuilt (or is simply newer than the target) then the target will also be rebuilt.

Now, the order-only prerequisites do item 1, i.e. impose build-order, but not item 2.