wmaker: make the '--replace' de-activable at compile time

As it is really unlikely that in normal use case someone would need this
feature, it is now conditional code, which is not enabled by default; the
configure scripts now propose a '--enable-wmreplace' option to enable the
corresponding code, as people making package for distributions may want to
enable the feature to provide users the ability to give a try of all the
window managers.

Suggested-by: Carlos R. Mafra <crmafra@gmail.com>
Signed-off-by: Christophe CURIS <christophe.curis@free.fr>

src/screen.h

@@ -69,7 +69,9 @@ typedef struct WDrawerChain {
 typedef struct _WScreen {
     int	screen;			       /* screen number */
     Window info_window;		       /* for our window manager info stuff */
+#ifdef USE_ICCCM_WMREPLACE
     Atom sn_atom;		       /* window manager selection */
+#endif
 
     int scr_width;		       /* size of the screen */
     int scr_height;