Merge changes Ie1194452,I9c42191b,I340deefa,I313493fa,Ia900dab4

* changes:
  Use arch-specific flags to fix multilib build.
  Tweak makefile
  Merge seek support from 1.2 to 1.6
  Update libpng to version 1.6.3
  Revert "Make png_uint_32 32 bits wide on all platforms."