Recursive `make` and `-j`

When using recursive make invocations with the `-j` flag for parallel execution, jobserver communication between parent and child make processes can break down, causing loss of parallelism and warnings. The issue can be fixed by using the `+` prefix or `$(MAKE)` variable instead of plain `make` in recipes, though behavior varies across GNU Make versions (3.81 to 4.3). When explicit `-j` options are specified in recursive recipes, they override the parent's jobserver settings, potentially reducing or increasing parallelism unexpectedly.