[tlbuild] Bug in libpng affecting Solaris 10

[Mojca Miklavec]

Hi,

There is a bug in libpng that is preventing a successful build of TL
sources on Solaris 10 (both sparc and intel) with the latest GCC:
    https://sourceforge.net/p/libpng/bugs/245/

I believe that it was Apostolos who suggested me to use
    export CPPFLAGS="-D_XOPEN_SOURCE=600 -D__EXTENSIONS__"
to build libpng and that worked for me at least.

In case that the bug doesn't get fixed before the TL 2016 release, can
someone perhaps add these flags (only for libpng and only for Solaris)
as a temporary workaround?

This will have to be fixed sooner or later in one way or another, so
it might be better to address it now rather than in the last minute.

Thank you,
    Mojca