Attempt to update libpng

libpng is is a dependency for cairo, but cairo is rarely used within
AOO and if required it is usually brought in through the system cairo.
We don't even carry libpng in the dependency list.

Updating this library is mostly innocuous but considering
CVE-2015-8540 it is convenient to keep our copy updated JIC.

libpng/makefile.mk

@@ -38,10 +38,10 @@ all:
 
 # --- Files --------------------------------------------------------
 
-LIBPNGVERSION=1.5.1
+LIBPNGVERSION=1.5.26
 
 TARFILE_NAME=$(PRJNAME)-$(LIBPNGVERSION)
-TARFILE_MD5=220035f111ea045a51e290906025e8b5
+TARFILE_MD5=3ca98347a5541a2dad55cd6d07ee60a9
 
 PATCH_FILES=$(PRJNAME)-$(LIBPNGVERSION).patch
 ADDITIONAL_FILES=makefile.mk pnglibconf.h