Single-header libraries like this require passing a bunch of defines _once_
before including the header, but not multiple times. This can make it tricky
in user code to know when to request the implementation, if the header needs
to be included in multiple files.
So properly compiling a .cpp file for the implementation solves this, and
also enables us to properly disable warnings on thirdparty implementation code.
Thaddeus Crews
Ithamar R. Adema
Rémi Verschelde
Aaron Franke
Vincent