[libpng14] Changed AM_CONFIG_HEADER(config.h) to AC_CONFIG_HEADERS([config.h])

in configure.ac
[libpng14]Changed default value of PNG_USER_CACHE_MAX from 0 to 32767

in pngconf.h.
 [libpng14] Imported from libpng-1.4.13beta02.tar
[libpng14] Bump version to 1.4.13beta03
[libpng14] Avoid a possible memory leak in contrib/gregbook/readpng.c
[libpng14] Updated GIT URL in the manual.