reported by clang scan-build.

This reverts to the previous 'static' implementation and works round
the 'unused static function' warning by using PNG_UNUSED().

configurations don't build because of missing contrib/libtests test programs
and overly complex dependencies in scripts/pnglibconf.dfa. This change adds
contrib/conftest/*.dfa files that can be used in automatic build
scripts to ensure that these configurations continue to build.

in pngminim.

to PNG_SET_UNKNOWN_SUPPORTED in pngpread.c; it led to the possibility of
calling png_handle_unknown() twice.

the #ifdef PNG_SET_UNKNOWN_CHUNKS_SUPPORTED/#endif block.
Removed recently-added #ifdef from pngpread.c; moving the prototype
in pngpriv.h fixed the compiler warning that the #ifdef was trying
to solve.

ifdef PNG_SET_UNKNOWN_CHUNKS_SUPPORTE in pngpread.c to agree with
what is in pngpriv.h

(#ifdef PNG_HANDLE_AS_UNKNOWN_SUPPORTED/#endif in pngpread.c)

ctype char checking macros take an unsigned char argument, not a signed char.

or intrinsics to be turned on during the build by defining
PNG_ARM_NEON_IMPLEMENTATION to the correct value (2 or 1).  The
default is "undefined" and the build type is selected in pngpriv.h.

We use -mfpu=neon instead of a configuration flag.

pngpriv.h and detect the broken GCC compilers.

it can manage, limited by the number of rows.

Use filter_neon_intrinsics.c in preference to filter_neon.S when possible.

pngrtran.c