Revert "Hardcoding xsessions directory to /usr/share/xsessions as suggested in #317"

This reverts commit f0ad1117ba.

Makefile

@@ -65,13 +65,13 @@ endif
 	mkdir -p ${DESTDIR}${MANPREFIX}/man1
 	sed "s/VERSION/${VERSION}/g" < dwm.1 > ${DESTDIR}${MANPREFIX}/man1/dwm.1
 	chmod 644 ${DESTDIR}${MANPREFIX}/man1/dwm.1
-	sudo mkdir -p /usr/share/xsessions
-	sudo cp -n dwm.desktop /usr/share/xsessions
-	sudo chmod 644 /usr/share/xsessions/dwm.desktop
+	mkdir -p ${DESTDIR}${PREFIX}/share/xsession
+	cp -n dwm.desktop ${DESTDIR}${PREFIX}/share/xsession
+	chmod 644 ${DESTDIR}${PREFIX}/share/xsession/dwm.desktop
 
 uninstall:
 	rm -f ${DESTDIR}${PREFIX}/bin/dwm\
 		${DESTDIR}${MANPREFIX}/man1/dwm.1\
-		/usr/share/xsessions/dwm.desktop
+		${DESTDIR}${PREFIX}/share/xsession/dwm.desktop
 
 .PHONY: all options clean dist install uninstall