At least in C, "header only" libraries typically require a #define LIBRARYNAME_IMPLEMENTATION somewhere in a .c file to define the symbols declared in the heade

At least in C, "header only" libraries typically require a #define LIBRARYNAME_IMPLEMENTATION somewhere in a .c file to define the symbols declared in the header. That way you're not including a copy of every function in every object file, which is what would slow down compile times. C compilers are very fast at skipping #ifdef blocks.