We compile zlib as own dependency, but are not informing BLOSC
to use it. This leads to zlib symbols defined twice when linking
Blender: one set comes from libz.a and another one from libblosc.a.
Tested on Linux Debian testing and CentOS 7.5.
It is possible that this change on its own will lead to linking
errors after libraries are re-compiled, This will be fixed as
a dedicated fix to Blender's build system.