[libpng17] Moved redefines of png_error(), png_warning(), png_chunk_error(),

and png_chunk_warning() from pngpriv.h to png.h to make them visible
to libpng-calling applications.
[libpng17] Moved OS dependent code from arm/arm_init.c, to allow the included

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.
[libpng17] Removed "#define PNG_NO_WARNINGS" from contrib/pngminim/*/pngusr.h

because warnings are already turned off in the *.dfa files.