1) Include it wherever it's used. The convention is to use the preprocessor[1] so it only appears once in the final binary

2) Most developers just want to compile to a single binary. Any developer who for some reason needs separately compiled objects should be able to quickly achieve that from a single-file library

[1] https://github.com/phoboslab/qoi/blob/324c2243b2fb35f741a88d...