progs: Fix linking of Xlib demos for non-autoconf builds

The Xlib demos were fixed to use $(X11_LIBS) so that configure could
detect the proper directory to link the library from, but this broke
the non-autoconf builds. Give X11_LIBS a default value to fallback on.

configs/default

@@ -119,7 +119,7 @@ APP_LIB_DEPS    = $(EXTRA_LIB_PATH) -L$(TOP)/$(LIB_DIR) -l$(GLUT_LIB) -l$(GLU_LI
 
 # Program dependencies - specific GL/glut libraries added in Makefiles
 APP_LIB_DEPS = -lm
-
+X11_LIBS = -lX11
 
 # Installation directories (for make install)
 INSTALL_DIR = /usr/local