Blender Git Loki

CMake: Fix libpng library path

The path in the precompiled libraries is libpng16.a rather than
libpng.a.

CMake: Linux, Hard-code all paths to pre-compiled libraries

Due to lack of standard in how FindFOO modules work, and, possibly, how
CMake's find helpers work as well, it was possible that system-wide
libraries are used.

This change ensures that all libraries are used from their pre-compiled
location.