Update CMakeLists.txt to use Sourcefiles

This establishes (near) parity of source code file lists
between the Makefile and CMakeLists.txt

To make that change I messed around CMakeLists.txt a bit.
It now uses target_sources and target_compile_definitions.
I also removed some MSVC stuff since we don't actually
care about MSVC--it made things easier.

CMake minimum version 3.0 -> 3.13 for target_sources.