simplify install target

no need to abuse the pwd shell command, we have a handy DESTDIR
variable, which the makefile should respect too. but because we need
to pass PREFIX anyways (since we need to override the correct upstream
default, /usr/local) it's probably not worth implementing DESTDIR in
the Makefile...

debian/rules

@@ -8,4 +8,4 @@
 	dh $@
 
 override_dh_auto_install:
-	$(MAKE) PREFIX=$$(pwd)/debian/vmtouch/usr install
+	$(MAKE) PREFIX=$(CURDIR)/debian/vmtouch/usr install