Even if Blackfin GNU toolchain add prefix '_' to all symbols,
symbol prefix is not accepted in the link flag --version-script.
Don't append prefix in the symbols in the version script file.

--- libpng-1.6.8/Makefile.am	2013-12-30 18:23:08.261797417 +0800
+++ libpng-1.6.8.bak/Makefile.am	2013-12-30 18:22:15.565795726 +0800
@@ -225,7 +225,7 @@
 # interfering with the symbol file format.
 SYMBOL_CFLAGS = -DPNGLIB_LIBNAME='PNG@PNGLIB_MAJOR@@PNGLIB_MINOR@_0'\
 		-DPNGLIB_VERSION='@PNGLIB_VERSION@'\
-		-DSYMBOL_PREFIX='$(SYMBOL_PREFIX)'\
+		-DSYMBOL_PREFIX=''\
 		-DPNG_NO_USE_READ_MACROS -DPNG_BUILDING_SYMBOL_TABLE
 
 if DO_PNG_PREFIX
--- libpng-1.6.8/Makefile.in	2013-12-30 18:23:08.261797417 +0800
+++ libpng-1.6.8.bak/Makefile.in	2013-12-30 18:22:09.853794547 +0800
@@ -698,7 +698,7 @@
 # interfering with the symbol file format.
 SYMBOL_CFLAGS = -DPNGLIB_LIBNAME='PNG@PNGLIB_MAJOR@@PNGLIB_MINOR@_0' \
 	-DPNGLIB_VERSION='@PNGLIB_VERSION@' \
-	-DSYMBOL_PREFIX='$(SYMBOL_PREFIX)' -DPNG_NO_USE_READ_MACROS \
+	-DSYMBOL_PREFIX='' -DPNG_NO_USE_READ_MACROS \
 	-DPNG_BUILDING_SYMBOL_TABLE $(am__append_5)
 all: $(BUILT_SOURCES) config.h
 	$(MAKE) $(AM_MAKEFLAGS) all-am