[libpng17] Removed or marked PNG_UNUSED some harmless "dead assignments"

reported by clang scan-build.

ANNOUNCE

@@ -1,5 +1,5 @@
 
-Libpng 1.7.0beta24 - November 24, 2013
+Libpng 1.7.0beta24 - November 25, 2013
 
 This is not intended to be a public release.  It will be replaced
 within a few weeks by a public version or by another test version.
@@ -425,17 +425,21 @@ Version 1.7.0beta23 [November 24, 2013]
   Moved prototype for png_handle_unknown() in pngpriv.h outside of
     the #ifdef PNG_SET_UNKNOWN_CHUNKS_SUPPORTED/#endif block.
   Added "-Wall" to CFLAGS in contrib/pngminim/*/makefile
+  Conditionally compile some unused functions reported by -Wall in
+    pngminim.
   Fixed 'minimal' builds. Various obviously useful minimal configurations
-    don't build because of missing contrib/libtests test programs and overly
-    complex dependencies in scripts/pnglibconf.dfa. This change adds
-    contrib/conftest/*.dfa files that can be used in automatic build
+    don't build because of missing contrib/libtests test programs and
+    overly complex dependencies in scripts/pnglibconf.dfa. This change
+    adds contrib/conftest/*.dfa files that can be used in automatic build
     scripts to ensure that these configurations continue to build.
   Enabled WRITE_INVERT and WRITE_PACK in contrib/pngminim/encoder.
   Fixed pngvalid 'fail' function declaration on the Intel C Compiler.
     This reverts to the previous 'static' implementation and works round
     the 'unused static function' warning by using PNG_UNUSED().
 
-Version 1.7.0beta24 [November 24, 2013]
+Version 1.7.0beta24 [November 25, 2013]
+  Removed or marked PNG_UNUSED some harmless "dead assignments" reported
+    by clang scan-build.
 
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