[libpng12] Changed AM_CONFIG_HEADER(config.h) to AC_CONFIG_HEADERS([config.h])

in configure.ac
[libpng12] Changed default value of PNG_USER_CACHE_MAX from 0 to 32767

in pngconf.h.
[libpng12] Imported from libpng-1.2.51beta01.tar
[libpng12] Bump version to 1.2.51beta02
[libpng12] Avoid a possible memory leak in contrib/gregbook/readpng.c
[libpng12] Update GIT URL in the manual.