Fix libpng properly (forgot this in a previous commit).

extras/contrib/src/Patches/libpng-1.2.8-makefile.patch

+--- libpng-1.2.8/scripts/makefile.linux	2004-12-03 01:14:31.000000000 +0100
++++ libpng/scripts/makefile.linux	2006-07-14 10:12:00.000000000 +0200
+@@ -13,7 +13,7 @@
+ # where "make install" puts libpng12.a, libpng12.so*,
+ # libpng12/png.h and libpng12/pngconf.h
+ # Prefix must be a full pathname.
+-prefix=/usr/local
++prefix=$(PREFIX)
+ 
+ # Where the zlib library and include files are located.
+ #ZLIBLIB=/usr/local/lib
+@@ -53,7 +53,7 @@
+ # If you're going to install into a temporary location
+ # via DESTDIR, $(DESTDIR)$(prefix) must already exist before
+ # you execute make install.
+-DESTDIR=
++#DESTDIR=
+ 
+ DB=$(DESTDIR)$(BINPATH)
+ DI=$(DESTDIR)$(INCPATH)