[libpng15] Revised stack marking in arm/filter_neon.S and configure.ac
[libpng15] Ensure that NEON filter stuff is completely disabled when

switched 'off'.  Previously the ARM NEON specific files were still built
if the option was switched 'off' as opposed to being explicitly disabled.
[libpng15] Avoid a compiler warning about unused png_ptr

in translate_gamma_flags()