Ninja makeserver support
Description of problem:
Building `qemu` using a patched version of `ninja`[0] to utilize `make`'s jobserver feature doesn't work when building `qemu`. Usually, when using a jobserver to control the number of jobs being built in parallel across multiple different builds (i.e. when building with `open-embedded` or `buildroot`), the `-j$(nproc)` argument is left out. In this case, the `Qemu` `Makefile` interprets the absent `-j` argument as a wish for a single process only, and adds a `-j1` argument to the `ninja` call.
Steps to reproduce:
1. Built/install the patched `ninja` from [0]: `export PATH=<path/to/ninja>:$PATH`
2. Start the attached [jobserver.py](/uploads/8215e8a470c97cd456d2d14e2c71c6a5/jobserver.py) script: `python jobserver.py /tmp/jobserver 4`
3. Configure `qemu`: `mkdir build; ../configure`
4. Build `qemu`: `MAKEFLAGS="--jobserver-auth=fifo:/tmp/jobserver" make`
5. Observe that only a single CPU/core is being used.

Now, to avoid passing `-j1` to `ninja`, remove filtering of `-j` arguments from the `Makefile`:

```patch
diff --git a/Makefile b/Makefile
index bfc4b2c8e9..d66141787e 100644
--- a/Makefile
+++ b/Makefile
@@ -142,7 +142,6 @@ MAKE.k = $(findstring k,$(firstword $(filter-out --%,$(MAKEFLAGS))))
 MAKE.q = $(findstring q,$(firstword $(filter-out --%,$(MAKEFLAGS))))
 MAKE.nq = $(if $(word 2, $(MAKE.n) $(MAKE.q)),nq)
 NINJAFLAGS = $(if $V,-v) $(if $(MAKE.n), -n) $(if $(MAKE.k), -k0) \
-        $(filter-out -j, $(lastword -j1 $(filter -l% -j%, $(MAKEFLAGS)))) \
         -d keepdepfile
 ninja-cmd-goals = $(or $(MAKECMDGOALS), all)
 ninja-cmd-goals += $(foreach g, $(MAKECMDGOALS), $(.ninja-goals.$g))
```

Run the build again, and see four jobs being run in parallel:

`make clean; MAKEFLAGS="--jobserver-auth=fifo:/tmp/jobserver" make`
Additional information:
[0] https://github.com/stefanb2/ninja/tree/topic-issue-1139-part-3-jobserver-fifo