Use the built-in define __linux__ instead of the manually set LINUX
Don't define MACOS and LINUX

Nothing within the library actually uses these defines for anything
any more.

If necessary, the built-in defines __APPLE__ and __linux__ can be
used instead of these manually added defines. This is similar to how
we use _WIN32 instead of manually defining WIN32.
Merge pull request #822 from mstorsjo/simplify-platform-defines

Don't manually define MACOS and LINUX, use built-in defines instead