graphics/libmng: upgrade from 1.0.10 to 2.0.3

Despite the seemingly large version jump, the sole change is
the compatibility with lcms-2.x.

I'm now also adding fixes for some compiler-warnings raised
by the modern clang. These suggest, certain parts of the code
aren't even really excercised.

Neither API nor ABI are changing, so no shared library
version-bump.