Experimental hotfixes to Hitcoder's Linux libpng problems mentioned on the MB

I found out the following things after doing a bit of research, and made attempts to support these (if not using the libs/ copy of libpng, which is 1.2.46 with a apng patch):
* PNG_EXPORT() was modified in libpng 1.5.0 to take four args instead of two
* png_strlen, png_memcmp, png_memcpy and png_memset were made private in 1.5.0 and then eventually removed in 1.6.0 (I'm not sure whether I need to define them for 1.5.0 too please help me out here)

No changes should occur for Windows builds (at least, for now), I can confirm it compiles fine on my end