that don't do alignment correctly.

implementation of the ARM NEON discovery function to be set at
build-time and provide sample implementations from the current code in the
contrib/arm-neon subdirectory. The __linux__ code has also been changed to
compile and link on Android by using /proc/cpuinfo, and the old linux code
is in contrib/arm-neon/linux-auxv.c.  The new code avoids POSIX and Linux
dependencies apart from opening /proc/cpuinfo and is C90 compliant.

requirements of png_modifier are greater than that of png_store and as a
consequence compilation of pngvalid.c results in a warning about increased
alignment requirements because of the bare cast to (png_modifier*). The code
is safe, because the pointer is known to point to a stack allocated
png_modifier, but this change avoids the warning.

  Fixed default behavior of ARM_NEON_API. If the ARM NEON API option was
    compiled without the CHECK option it defaulted to on, not off.

  Check user callback behavior in pngunknown.c. Previous versions compiled
    if SAVE_UNKNOWN was not available but did nothing since the callback
    was never implemented.

  Merged pngunknown.c with 1.7 version and back ported 1.7 improvements/fixes