Install manual pages to the correct location.

Index: Makefile
--- Makefile.orig
+++ Makefile
@@ -63,7 +63,7 @@ install:
 	mkdir -p $(DESTDIR)$(DATADIR)/metainfo/
 	mkdir -p $(DESTDIR)$(DATADIR)/applications/
 	mkdir -p $(DESTDIR)$(DATADIR)/icons/
-	mkdir -p $(DESTDIR)$(DATADIR)/man/man6/
+	mkdir -p $(DESTDIR)$(PREFIX)/man/man6/
 	install -m 755 $(BINARIES) $(DESTDIR)$(BINDIR)/
 	install -m 644 data/jumpbump.dat \
 		$(DESTDIR)$(GAMEDATADIR)/jumpnbump/jumpbump.dat
@@ -73,7 +73,7 @@ install:
 		$(DESTDIR)$(DATADIR)/applications/jumpnbump.desktop
 	install -m 644 dist/jumpnbump.png \
 		$(DESTDIR)$(DATADIR)/icons/jumpnbump.png
-	install -m 644 dist/jumpnbump.6 $(DESTDIR)$(DATADIR)/man/man6/
+	install -m 644 dist/jumpnbump.6 $(DESTDIR)$(PREFIX)/man/man6/
 
 #	$(MAKE) -C menu install
 
