0x8000 flag definitions on 16-bit systems. They aren't supported
yet the defs *probably* work, however it seems much safer to do this
and be advised if anyone, contrary to advice, is building libpng 1.6
on a 16-bit system. It also adds back various switch default clauses
for GCC; GCC errors out if they are not present (with an appropriately
high level of warnings).

in pngread.c

pngrtran.c to suppress clang warnings (Bug report by Viktor Szakats).

"libpng_autotools_extra" list in autogen.sh because it was causing a
false positive for missing files (bug report by Robert C. Seacord).

fixed potential leak of png_pixels in contrib/pngminus/pnm2png.c

(Bug report by Viktor Szaka'ts).  Several warnings remain and are
unavoidable, where we test for overflow.

in libpng17.

manual pages (Bug report by Arc Riley).

to png.h to avoid compatibility warnings.

preserve API compatibility, the new defines all default to "extern"
(requested by Jan Nijtmans).