Better configure options for udev dir and rules

Allows passing --with-udev=DIR and/or --with-udev-rules=NAME to specify
the directory where udev is installed and the name of the rules file.
Now mtp-probe will be installed as @UDEV@/mtp-probe and the rules file
will be at @UDEV@/rules.d/@UDEV_RULES@.
diff --git a/Makefile.am b/Makefile.am
index 200d5f8..26259c0 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -9,16 +9,16 @@
 # This stuff only makes sense on Linux so only
 # build and ship it on Linux.
 if USE_LINUX
-udevrulesdir=$(prefix)/@UDEV_RULES_PATH@
-udevrules_DATA=@UDEV_RULES_NAME@
+udevrulesdir=$(prefix)/@UDEV@/rules.d
+udevrules_DATA=@UDEV_RULES@
 noinst_DATA=libmtp.usermap libmtp.fdi
 
 libmtp.usermap: util/mtp-hotplug
 	util/mtp-hotplug > libmtp.usermap
-@UDEV_RULES_NAME@: util/mtp-hotplug
-	util/mtp-hotplug -u > @UDEV_RULES_NAME@
+@UDEV_RULES@: util/mtp-hotplug
+	util/mtp-hotplug -u -p"@UDEV@" > @UDEV_RULES@
 libmtp.fdi: util/mtp-hotplug
 	util/mtp-hotplug -H > libmtp.fdi
 
-CLEANFILES = libmtp.usermap @UDEV_RULES_NAME@ libmtp.fdi
+CLEANFILES = libmtp.usermap @UDEV_RULES@ libmtp.fdi
 endif