Set missing WM_COMMAND using pid given from libXRes

For apps which are not setting the window WM_COMMAND property like those old
apps using Motif toolkit (I am thinking of NEdit for example)
it's bringing some issues in windowmaker which is relying on it for a few interactions.
Especially,
*an app without WM_COMMAND will not be saved during the workspace state
(so session restore is not working for them)
*when added to the dock, the settings parameters are empty and need to be filled
*cannot autostart from the dock (even if the settings are manually filled and saved)
*right click on the app titlebar, and choosing Launch has no effect

Most of the time, those apps are also not setting the X11_NET_WM_PID property.
With the pid we could have a chance to find the running program.
To link a window to a pid, there is the X11 Resource extension library (libXRes).
After checking, gnome and xfce are also using the same method to handle such issues.

The patch is checking if the libXRes is present on the system (but it's not mandatory to compile).
Then, it adds a layer on top of wNETWMGetPidForWindow to not only check the window property
but if necessary to get the underlying pid from libXRes if available.
That's solving the points mentioned above.

configure.ac

@@ -580,6 +580,18 @@ AS_IF([test "x$enable_wmreplace" = "xyes"],
        [define to support ICCCM protocol for window manager replacement])
      supported_xext="$supported_xext WMReplace"])
 
+dnl XRes support
+dnl ==============
+m4_divert_push([INIT_PREPARE])dnl
+AC_ARG_ENABLE([res],
+    [AS_HELP_STRING([--disable-res], [disable resource window extension support])],
+    [AS_CASE(["$enableval"],
+        [yes|no], [],
+        [AC_MSG_ERROR([bad value $enableval for --enable-res]) ]) ],
+    [enable_res=auto])
+m4_divert_pop([INIT_PREPARE])dnl
+
+WM_XEXT_CHECK_XRES
 
 dnl XShape support
 dnl ==============