How to get GCC or CLANG to throw warnings when there is a missing function definition?

I'm coding in pure C.

I'm familiar with -Wmissing-prototypes which will issue a warning when the function prototype is missing.

How about one for when there is no function definition?

This would be useful while refactoring a library when in the process of adding/removing functions, you had a good idea for a new function, but forgot to implement it.