Add uninstall section to Makefile and update vars

Makefile

@@ -1,3 +1,12 @@
+PREFIX ?= /usr/local
+BINDIR = $(PREFIX)/bin
+MANDIR = $(PREFIX)/share/man
+
 install:
-	install -D tdrop $(DESTDIR)/usr/bin/tdrop
-	install -D -m 644 tdrop.groff $(DESTDIR)/usr/share/man/man1/tdrop.1
+	# 755 is default
+	install -D -m 755 tdrop "$(DESTDIR)$(BINDIR)"
+	install -D -m 644 tdrop.groff "$(DESTDIR)$(MANDIR)"/man1/tdrop.1
+
+uninstall:
+	rm -f "$(DESTDIR)$(BINDIR)"/tdrop
+	rf -rf "$(DESTDIR)$(MANDIR)"