[CMake] Define MAC_OSX for cmake builds on macOS
[CMake] Define MAC_OSX for cmake builds on macOS

A few places in our code rely on this precompiler definition, most
notably in the datadir and zk-params dir default values. Autotools sets
this automatically, but it is not passed into `pivx-config.h` as it
isn't necessary for autotools builds.

Instead of modify the autotools configure script to pass this define
into the config file, simply define it in `CMakeLists.txt`

Darknet-Crypto/Darknet,PIVX-Project/PIVX,PIVX-Project/PIVX,PIVX-Project/PIVX,PIVX-Project/PIVX,Darknet-Crypto/Darknet,PIVX-Project/PIVX,PIVX-Project/PIVX,Darknet-Crypto/Darknet,PIVX-Project/PIVX,PIVX-Project/PIVX,Darknet-Crypto/Darknet,Darknet-Crypto/Darknet,PIVX-Project/PIVX,Darknet-Crypto/Darknet