win32: don't run subprocess tests on Mingw32 platform

On Tue, Jun 14, 2016 at 04:44:57PM +0100, Daniel P. Berrange wrote:
> The g_test_trap_subprocess() method does not work on the
> Mingw32 platform, causing the test-qdev-global-props
> test case to abort
>
> (test-logging.exe:230): GLib-ERROR **: g_test_trap_subprocess()
> failed: Failed to execute helper program (No such file or directory)
>
> This failure was introduced a while ago in
>
>   commit 2177801a4899bf29108b3d471417a5b4d701ec29
>   Author: Eduardo Habkost <ehabkost@redhat.com>
>   Date:   Fri Aug 8 16:03:27 2014 -0300
>
>     test-qdev-global-props: Run tests on subprocess
>
> Modify the configure time check to avoid enabling this feature
> on Mingw, rather than trying to rewrite the test to avoid this
> feature.

I would do the following instead, just in case we have extra code
looking at $glib_subprocess one day.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
