[tlbuild] PPC builds

[Karl Berry]

Hi Johannes,

    Date: Sun, 16 Feb 2020 23:09:54 +0100
    From: Johannes Hielscher <jhielscher at posteo.de>
    
    ... I am mildly optimsitic that once the VSX flag is treated similar
    to the ARM NEON case, then the build creates a usable libpng and
    clears the way for successful native TL build for Linux on POWER9.

I imported the vsx stuff from libpng-src/{configure.ac,Makefile.am} to
TL's {configure.ac,Makefile.am} and ran autoreconf, in r54291.

I don't have any way to test it, beyond checking that it still builds
for me without any VSX stuff being enabled. Hope it flies ... --thanks, karl.