Make building minizip optional, if your linux distro supplies a minizip binary (e.g. in the zlib-bin package on debian) you might as well use it instead of building this one.
Note that this isn't implemented in the minizip CMakeLists.txt, but at the higher level (kicad) that includes the minizip subdirectory, so that the whole minizip directory can be dropped from the shipped source tree if it isn't needed on your distro. This makes the source package a fraction smaller and removes possible complications of having a different license on the minizip code to the rest of kicad.