Makefile: prepend DESTDIR to INSTALLDIRx variables

This allows the files to be first staged instead of being directly
installed into the path specified in INSTALLPREFIX.

The purpose is to allow the FreeBSD ports Makefile to cleanly install
the files into the stagedir without hacks.

Sponsored by:	The FreeBSD Foundation

Makefile

@@ -110,16 +110,16 @@ depend:
 	makedepend -f$(MAKEFILE_LIST) -Y $(OBJS:.o=.c) 2>/dev/null
 
 install:	$(HEADERS) $(STATICLIB) $(SHAREDLIB) natpmpc-shared natpmp.pc
-	$(INSTALL) -d $(INSTALLDIRINC)
-	$(INSTALL) -m 644 $(HEADERS) $(INSTALLDIRINC)
-	$(INSTALL) -d $(INSTALLDIRLIB)
-	$(INSTALL) -m 644 $(STATICLIB) $(INSTALLDIRLIB)
-	$(INSTALL) -m 644 $(SHAREDLIB) $(INSTALLDIRLIB)/$(SONAME)
+	$(INSTALL) -d $(DESTDIR)$(INSTALLDIRINC)
+	$(INSTALL) -m 644 $(HEADERS) $(DESTDIR)$(INSTALLDIRINC)
+	$(INSTALL) -d $(DESTDIR)$(INSTALLDIRLIB)
+	$(INSTALL) -m 644 $(STATICLIB) $(DESTDIR)$(INSTALLDIRLIB)
+	$(INSTALL) -m 644 $(SHAREDLIB) $(DESTDIR)$(INSTALLDIRLIB)/$(SONAME)
 	$(INSTALL) -d $(DESTDIR)$(PKGCONFIGDIR)
 	$(INSTALL) -m 644 natpmp.pc $(DESTDIR)$(PKGCONFIGDIR)
-	$(INSTALL) -d $(INSTALLDIRBIN)
-	$(INSTALL) -m 755 natpmpc-shared $(INSTALLDIRBIN)/natpmpc
-	ln -s -f $(SONAME) $(INSTALLDIRLIB)/$(SHAREDLIB)
+	$(INSTALL) -d $(DESTDIR)$(INSTALLDIRBIN)
+	$(INSTALL) -m 755 natpmpc-shared $(DESTDIR)$(INSTALLDIRBIN)/natpmpc
+	ln -s -f $(SONAME) $(DESTDIR)$(INSTALLDIRLIB)/$(SHAREDLIB)
 
 $(JNIHEADERS): fr/free/miniupnp/libnatpmp/NatPmp.class
 	$(JAVAH) -jni fr.free.miniupnp.libnatpmp.NatPmp
@@ -169,11 +169,11 @@ mvn_install:
 	 -DcreateChecksum=true
 
 cleaninstall:
-	$(RM) $(addprefix $(INSTALLDIRINC), $(HEADERS))
-	$(RM) $(INSTALLDIRLIB)/$(SONAME)
-	$(RM) $(INSTALLDIRLIB)/$(SHAREDLIB)
-	$(RM) $(INSTALLDIRLIB)/$(STATICLIB)
-	$(RM) $(INSTALLDIRLIB)/$(PKGCONFIGDIR)
+	$(RM) $(addprefix $(DESTDIR)$(INSTALLDIRINC)/, $(HEADERS))
+	$(RM) $(DESTDIR)$(INSTALLDIRLIB)/$(SONAME)
+	$(RM) $(DESTDIR)$(INSTALLDIRLIB)/$(SHAREDLIB)
+	$(RM) $(DESTDIR)$(INSTALLDIRLIB)/$(STATICLIB)
+	$(RM) $(DESTDIR)$(PKGCONFIGDIR)
 
 natpmp.pc: VERSION
 	$(RM) $@