avoid mulitple redefinition when include from different .c/.cpp files.

[booknouse]

when #include "headers/codecfactory.h" from two cpp in a project, meet "multiple definition of xxx"

[lemire]

This PR does not appear to pass our unit tests.

[lemire]

Can you sync with our main branch? I added a few more CI tests.

Note that I cannot merge a pull request that fails our tests.