mingw: Require Fedora

Although it was never formally specified, it was always expected
that the mingw RPM build would happen on Fedora, if anything
because RHEL / CentOS don't ship the necessary mingw dependencies.

Make this fact explicit by erroring out if that's not the case,
the same way we already do in the main spec file.
Signed-off-by: NAndrea Bolognani <abologna@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>

mingw-libvirt.spec.in

 %{?mingw_package_header}
 
+# This spec file assumes you are building on a Fedora version
+# that's still supported by the vendor. It may work on other distros
+# or versions, but no effort will be made to ensure that going forward.
+%define min_fedora 26
+
+%if 0%{?fedora} && 0%{?fedora} >= %{min_fedora}
+    %define supported_platform 1
+%else
+    %define supported_platform 0
+%endif
+
 # Default to skipping autoreconf.  Distros can change just this one line
 # (or provide a command-line override) if they backport any patches that
 # touch configure.ac or Makefile.am.
@@ -124,6 +135,11 @@ MinGW Windows libvirt virtualization library, static version.
 %setup -q -n libvirt-%{version}
 
 %build
+%if ! %{supported_platform}
+echo "This RPM requires Fedora >= %{min_fedora}"
+exit 1
+%endif
+
 %if ! %{with_phyp}
     %define _without_phyp --without-phyp
 %endif