1888: `make check` fails on Windows 10 r=msiglreith a=bugabinga

* GNU Make 4.2
* Windows 10
* PowerShell

```ps
PS C:\Users\okr\gfx> make check
#Note: excluding `warden` here, since it depends on serialization
process_begin: CreateProcess(NULL, #Note: excluding `warden` here, since it depends on serialization, ...) failed.
make (e=2): Das System kann die angegebene Datei nicht finden.
make: *** [makefile:49: check] Error 2
```

Fixes #issue
PR checklist:
- [ ] `make` succeeds (on *nix)
- [ ] `make reftests` succeeds
- [ ] tested examples with the following backends: