Revert "win32: don't run subprocess tests on Mingw32 platform"

This reverts commit 7ad9339e.

The error "Failed to execute helper program (No such file or directory)"
is due to broken glib installation, missing windows gspawn helpers.
Signed-off-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20170104205722.26492-1-marcandre.lureau@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

configure

@@ -3079,7 +3079,7 @@ fi
 
 # g_test_trap_subprocess added in 2.38. Used by some tests.
 glib_subprocess=yes
-if test "$mingw32" = "yes" || ! $pkg_config --atleast-version=2.38 glib-2.0; then
+if ! $pkg_config --atleast-version=2.38 glib-2.0; then
     glib_subprocess=no
 fi