Should use the distributed zlib if none is available on target

[thoni56]

c-xrefactory requires zlib but currently a build fails if none is available on the target system. This is particularly problematic now with the easy el-get install which basically says that the make command failed.

But c-xrefactory sources includes a very early, but sufficient, version of zlib which could be used in case the system does not provide one. This would lift yet another burdon from the users shoulders.