[PATCH, Makefile.in] refine selftest recipes to restore mingw bootstrap

[Olivier Hainque]

> On Nov 28, 2017, at 11:56 , Tamar Christina <Tamar.Christina@arm.com> wrote:
>> For gcc in particular, ISTM it depends on the target for which you configured.
> 
> This is true, but In this case, the tool should have never seen "/dev/null". Since the invocation
> Of the command that uses SELFTEST_FLAGS should have converted it. Unless you're using
> Something like mingw32-make, which would explain the difference.

Oh, right. We are using

  GNU Make 3.82.90
  Built for i686-w64-mingw32

>> Maybe yours are configured for cygwin, not mingw (are they ?), and then are
>> linked with cygwin libraries which perform various transformations.
> 
> No, mine are configured for x86_64-w64-mingw32 and are just native gcc builds.
> It's just build using cygwin derived tools to not encounter certain differences like this one.
> 
> I think using nul is fine if it makes your use-case work, it shouldn't break the mingw64 builds.

OK. It does help our use case for sure.

Thanks for your feedback!

Olivier