That's not true in this case: if you look at the .c file you can see it actually has no definitions by default and is a relatively normal .h file when it's incl

That's not true in this case: if you look at the .c file you can see it actually has no definitions by default and is a relatively normal .h file when it's included. You can #include it in any number of files as long as exactly one has #define OLIVEC_IMPLEMENTATION.