Fixes and cleanups

[lighterowl]

I've never been a huge fan of custom platform defines which need to be supplied by the build system, especially when these defines can be replaced by checks relying on the presence/value of standard defines which are supplied by the operating system headers and/or the compiler :

• _WIN32 is defined for all 32- and 64-bit Windows environments, and can thus replace WIN32.
• The __linux__ define is set to 1 on systems using the Linux kernel, and can thus replace LINUX.
• MacOS can be identified by checking whether both __APPLE__ and __MACH__ are defined - such a check can replace MAC.

As a bonus, remove all unused source and header files from the tree. I don't think moving stuff to unused serves any purpose anymore.

This was tested on Windows (MSVC and MSYS2), Linux (GCC), and MacOS via osxcross.