Exclude systemd unit files for Flatpak builds

While building gnome-maps RPM to construct Flatpak[0], we came
across evolution-data-server installing systemd unit files. That
isn't supported by Flatpak at the moment[1].

The {?flatpak} macro is provided by flatpak-rpm-macros[2].

Similar patches have been applied to other GNOME components that
are being converted, such as:
https://src.fedoraproject.org/rpms/spice-gtk/c/a6a6e6cd1e

[0] https://docs.fedoraproject.org/en-US/flatpak/tutorial/
[1] https://github.com/flatpak/flatpak/issues/2787
[2] https://src.fedoraproject.org/rpms/flatpak-rpm-macros

evolution-data-server.spec

@@ -70,7 +70,9 @@ BuildRequires: libdb-devel
 BuildRequires: perl-generators
 BuildRequires: vala
 BuildRequires: sendmail
+%if 0%{?flatpak}
 BuildRequires: systemd
+%endif
 
 BuildRequires: pkgconfig(gcr-3)  >= %{gcr_version}
 BuildRequires: pkgconfig(gcr-base-3)  >= %{gcr_version}
@@ -236,6 +238,9 @@ export CFLAGS="$RPM_OPT_FLAGS -DLDAP_DEPRECATED -fPIC -I%{_includedir}/et -Wno-d
 	-DENABLE_INTROSPECTION=ON \
 	-DENABLE_VALA_BINDINGS=ON \
 	-DENABLE_INSTALLED_TESTS=ON \
+        %if 0%{?flatpak}
+        -DWITH_SYSTEMDUSERUNITDIR=OFF \
+        %endif
 	%ldap_flags %krb5_flags %ssl_flags \
 	%largefile_flags %gtkdoc_flags \
 	..
@@ -325,10 +330,12 @@ find $RPM_BUILD_ROOT -name '*.so.*' -exec chmod +x {} \;
 %{_datadir}/dbus-1/services/org.gnome.evolution.dataserver.UserPrompter.service
 %{_datadir}/pixmaps/evolution-data-server
 
+%if ! 0%{?flatpak}
 %{_userunitdir}/evolution-addressbook-factory.service
 %{_userunitdir}/evolution-calendar-factory.service
 %{_userunitdir}/evolution-source-registry.service
 %{_userunitdir}/evolution-user-prompter.service
+%endif
 
 %dir %{_libdir}/evolution-data-server
 %dir %{credential_modules_dir}