Check that X11RDPBASE is set, give it a better description

Use BINBASE for the binary install path.

xorg/X11R7.6/rdp/Makefile

@@ -1,6 +1,9 @@
+# X11RDPBASE is the top-level X11rdp install directory
+ifeq ($(X11RDPBASE),)
+$(error X11RDPBASE needs to be set)
+endif
 
-#X11RDPBASE is an environment variable that needs to be set
-
+BINBASE = $(X11RDPBASE)/bin
 INCBASE = $(X11RDPBASE)/include
 LIBBASE = $(X11RDPBASE)/lib
 
@@ -113,4 +116,4 @@ fbcmap_mi.o: ../build_dir/xorg-server-1.9.3/fb/fbcmap_mi.c
 	$(CC) $(CFLAGS) -c ../build_dir/xorg-server-1.9.3/fb/fbcmap_mi.c
 
 install: all
-	$(INSTALL) X11rdp $(X11RDPBASE)/bin/X11rdp
+	$(INSTALL) X11rdp $(BINBASE)/X11rdp