configure: add helper for setting xserver directory

With this the X server directory can become independent from the
installation prefix set by autoconf.

Signed-off-by: Tiago Vignatti <tiago.vignatti@intel.com>

compositor/Makefile.am

@@ -3,7 +3,7 @@ bin_PROGRAMS = wayland-compositor
 AM_CPPFLAGS =					\
 	-DDATADIR='"$(datadir)"'		\
 	-DMODULEDIR='"$(moduledir)"'		\
-	-DBINDIR='"$(bindir)"'			\
+	-DXSERVER_PATH='"@XSERVER_PATH@"'	\
 	$(COMPOSITOR_CFLAGS)
 
 wayland_compositor_LDFLAGS = -export-dynamic

compositor/xserver-launcher.c

@@ -501,8 +501,8 @@ wlsc_xserver_handle_event(int listen_fd, uint32_t mask, void *data)
 		snprintf(logfile, sizeof logfile,
 			 "/tmp/x-log-%d", mxs->display);
 
-		if (execl(BINDIR "/Xorg",
-			  BINDIR "/Xorg",
+		if (execl(XSERVER_PATH,
+			  XSERVER_PATH,
 			  display,
 			  "-wayland",
 			  "-rootless",