build: install a login manager session entry

This makes it possible to run Weston session from GDM (and possibly
other login managers).

Makefile.am

@@ -156,6 +156,11 @@ endif # BUILD_WESTON_LAUNCH
 pkgconfigdir = $(libdir)/pkgconfig
 pkgconfig_DATA = src/weston.pc
 
+EXTRA_DIST += $(wayland_session_DATA)
+
+wayland_sessiondir = $(datadir)/wayland-sessions
+wayland_session_DATA = src/weston.desktop
+
 westonincludedir = $(includedir)/weston
 westoninclude_HEADERS =				\
 	src/version.h				\

src/weston.desktop

@@ -0,0 +1,5 @@
+[Desktop Entry]
+Name=Weston
+Comment=The reference Wayland server
+Exec=dbus-launch --exit-with-session weston
+Type=Application