[png-mng-implement] lilbpng-1.6.31beta01 is available

[Glenn Randers-Pehrson]

libpng-1.6.31beta01 is available from
https://ftp-osl.osuosl.org/pub/libpng/src/beta/libpng16
ftp://ftp-osl.osuosl.org/pub/libpng/src/beta/libpng16
http://libpng.download/src/beta/libpng16
and from
http://libpng.sf.net

Version 1.6.31beta01 [July 5, 2017]
Guard the definition of _POSIX_SOURCE in pngpriv.h (AIX already defines it;
bug report by Michael Felt).
Revised pngpriv.h to work around failure to compile arm/filter_neon.S
("typedef" directive is unrecognized by the assembler). The problem
was introduced in libpng-1.6.30beta01.
Added "Requires: zlib" to libpng.pc.in (Pieter Neerincx).
Added special case for FreeBSD in arm/filter_neon.S (Maya Rashish).

Glenn