SNOW-737 FIXED Version agnostic libPNG linking

Patch from http://jira.secondlife.com/secure/attachment/41125/version_agnostic_libpng__1.diff
applied with fuzz 3:

	patching file indra/cmake/PNG.cmake
	Hunk #1 succeeded at 6 with fuzz 3 (offset -2 lines).
	patching file indra/llimage/llpngwrapper.cpp
	Hunk #1 succeeded at 215 (offset -2 lines).
	Hunk #2 succeeded at 363 (offset -2 lines).
	patching file indra/llimage/llpngwrapper.h
	Hunk #1 succeeded at 31 (offset -2 lines).

Added entry in doc/contributions.txt.
No further changes other than that.

doc/contributions.txt

@@ -68,6 +68,7 @@ Blakar Ogre
 blino Nakamura
 	VWR-17
 Boroondas Gupte
+	SNOW-737
 	VWR-233
 	WEB-262
 bushing Spatula

indra/cmake/PNG.cmake

@@ -6,6 +6,6 @@ set(PNG_FIND_REQUIRED ON)
 if (STANDALONE)
   include(FindPNG)
 else (STANDALONE)
-  set(PNG_LIBRARIES png12)
+  set(PNG_LIBRARIES png)
   set(PNG_INCLUDE_DIRS ${LIBS_PREBUILT_DIR}/include)
 endif (STANDALONE)

indra/llimage/llpngwrapper.cpp

@@ -215,7 +215,7 @@ void LLPngWrapper::normalizeImage()
 	}
     if (mColorType == PNG_COLOR_TYPE_GRAY && mBitDepth < 8)
 	{
-		png_set_gray_1_2_4_to_8(mReadPngPtr);
+		png_set_expand_gray_1_2_4_to_8(mReadPngPtr);
 	}
 	if (mColorType == PNG_COLOR_TYPE_GRAY
 		|| mColorType == PNG_COLOR_TYPE_GRAY_ALPHA)
@@ -363,7 +363,7 @@ void LLPngWrapper::releaseResources()
 {
 	if (mReadPngPtr || mReadInfoPtr)
 	{
-		png_destroy_read_struct(&mReadPngPtr, &mReadInfoPtr, png_infopp_NULL);
+		png_destroy_read_struct(&mReadPngPtr, &mReadInfoPtr, NULL);
 		mReadPngPtr = NULL;
 		mReadInfoPtr = NULL;
 	}

indra/llimage/llpngwrapper.h

@@ -31,7 +31,7 @@
 #ifndef LL_LLPNGWRAPPER_H
 #define LL_LLPNGWRAPPER_H
 
-#include "libpng12/png.h"
+#include "png.h"
 #include "llimage.h"
 
 class LLPngWrapper