[libpng15] Enclosed the new png_get_palette_max

in #ifdef PNG_GET_PALETTE_MAX_SUPPORTED block, and revised pnglibconf.h
and pnglibconf.h.prebuilt accordingly.
[libpng15] Fix inconsistencies in use of PNG_GET_PALETTE_MAX
Merge remote-tracking branch 'origin/libpng15' into libpng15-ssse3