Say I have a Haskell program or library that I\'d like to make accessible to non-Haskellers, potentially C programmers. Can I compile it to C using GHC and then distribute this
Even if you could, I wouldn't call it "C source". GHC can use C as part of its compilation system, but the generated C code is not even slightly readable. Even if it could be read and understood, it would make no sense to modify it because there is no way (apart from back-porting the changes into Haskell) to incorporate any modifications made by C hackers into future versions of your program.
The term "source" means the code that is written by a human and used to generate the program. In this case that is the Haskell. C generated by a compiler is not "source code", it is an intermediate representation.