Support the most recent released version of `fmt` (previously known as `cppformat`) available.
This revision also de-bundles the source, as we no longer need to modify it to get the library to work with `pyrogenesis`.
**Linux**/**BSD**: Users now need to have `fmt` installed from their distribution's package repository. Minimum required version of `fmt` is `4.0.0`.
**OSX**: The source is acquired and compiled (in `build-osx-libs.sh`), then included and linked automatically,
**Windows**: The relevant header files are retained and, along with a pre-built library, are to be the only thing bundled.