Maintain $(DESTDIR) when overriding $(MANPREFIX) & $(DOCDIR)

Makefile

@@ -77,11 +77,11 @@ install:
 	install extras/luakit.png $(DESTDIR)/usr/share/pixmaps/
 	install -d $(DESTDIR)/usr/share/applications
 	install extras/luakit.desktop $(DESTDIR)/usr/share/applications/
-	install -d $(INSTALLDIR)/share/man/man1/
-	install -m644 luakit.1 $(INSTALLDIR)/share/man/man1/
+	install -d $(MANPREFIX)/man1/
+	install -m644 luakit.1 $(MANPREFIX)/man1/
 
 uninstall:
-	rm -rf $(INSTALLDIR)/bin/luakit $(INSTALLDIR)/share/luakit $(INSTALLDIR)/share/man/man1/luakit.1
+	rm -rf $(INSTALLDIR)/bin/luakit $(INSTALLDIR)/share/luakit $(MANPREFIX)/man1/luakit.1
 	rm -rf /usr/share/applications/luakit.desktop /usr/share/pixmaps/luakit.png
 
 newline: options;@echo

config.mk

@@ -2,11 +2,14 @@
 # of the current commit.
 VERSION   ?= $(shell ./build-utils/getversion.sh)
 
-# Paths
 PREFIX     ?= /usr/local
-INSTALLDIR ?= $(DESTDIR)$(PREFIX)
-MANPREFIX  ?= $(DESTDIR)$(PREFIX)/share/man
-DOCDIR     ?= $(DESTDIR)$(PREFIX)/share/luakit/docs
+INSTALLDIR := $(DESTDIR)$(PREFIX)
+
+MANPREFIX  ?= $(PREFIX)/share/man
+MANPREFIX  := $(DESTDIR)$(MANPREFIX)
+
+DOCDIR     ?= $(PREFIX)/share/luakit/docs
+DOCDIR     := $(DESTDIR)$(DOCDIR)
 
 # Use the Just-In-Time compiler for lua (for faster lua code execution)
 # See http://luajit.org/ & http://luajit.org/performance.html for more