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) and included/linked automatically,
Windows: The relevant header files are retained, and along with a pre-built library (not actually included in the revision yet) are to be the only thing bundled.