internal 'prefix.h' file can only be generated correctly after
pnglibconf.h, however the dependency was not in Makefile.am.  The
symptoms are unpredictable depending on the order make chooses to
build pngprefix.h and pnglibconf.h, often the error goes unnoticed
because there is a system pnglibconf.h to use instead.

png_set_* APIs

png_set_unknown_chunks().

two test programs to generate and test a PNG which should have the problem.

Discontinued distributing libpng-1.7.0-1.6.0-diff.txt and similar.

by earlier versions of GCC fixed for Cygwin and Min/GW (which both use old GCCs.)
ARM support is enabled by default in zlib.props (unsupported by Microsoft) and
ARM compilation is made possible by deleting the check for x86. The test programs
cannot be run because they are not signed.

pnglibconf.h to control the defaults.