xfce4-session: optional support polkit

If polkit not in DISTRO_FEATURES, it failed
...
|ERROR: Nothing PROVIDES 'polkit' (but meta-openembedded/meta-xfce/recipes-xfce/
xfce4-session/xfce4-session_4.13.2.bb DEPENDS on or otherwise requires it)
|polkit was skipped: missing required distro feature 'polkit' (not in DISTRO_FEATURES)
|ERROR: Required build target 'meta-world-pkgdata' has no buildable providers.
|Missing or unbuildable dependency chain was: ['meta-world-pkgdata', 'xfce4-session', 'polkit']
...

Upstream optional support polkit:
https://github.com/xfce-mirror/xfce4-session/commit/560219ad500c73e0fc2c3d959f96bcb8e2a54556

Signed-off-by: Hongxu Jia <hongxu.jia@windriver.com>
Signed-off-by: Khem Raj <raj.khem@gmail.com>

meta-xfce/recipes-xfce/xfce4-session/xfce4-session_4.13.2.bb

@@ -11,7 +11,6 @@ DEPENDS = " \
     libsm \
     libxfce4ui \
     libxfce4util \
-    polkit \
     virtual/libx11 \
     xfconf \
 "
@@ -20,6 +19,9 @@ inherit xfce update-alternatives distro_features_check
 
 REQUIRED_DISTRO_FEATURES = "x11"
 
+PACKAGECONFIG ??= "${@bb.utils.filter('DISTRO_FEATURES', 'polkit', d)}"
+PACKAGECONFIG[polkit] = "--enable-polkit, --disable-polkit, polkit"
+
 SRC_URI += " \
     file://0001-configure.in-hard-code-path-to-iceauth.patch \
 "