Helper utilities for easier benchmarking in C
Nice bit of code!  Here are a few suggestions:


Initialize your variables when you declare them.  This helps shorten the code a bit and reduce possible bugs.
You need documenting comments within your code.  Especially since you are considering this as a library.
I would remove the NULL check on code in your execTime method, and document that a valid function pointer should be passed in.  I would expect the code to break if I tried to time NULL, not NULL to be returned back.  The standard library does this same thing with a few of their functions, like strlen().
Good job checking the return from malloc().
I would use the C11 standard function timespec_get() for your timing code.  It's a C standard (important!) function that has nanosecond precision.
If you want a better Fibonacci algorithm, here is an answer I wrote up a while ago on it.
Good job using puts() in place of printf() when you can.
You don't have to return 0 at the end of main(). The C standard  knows how frequently this is used, and lets us omit it.


  C99 &amp; C11 §5.1.2.2(3)
  
  ...reaching the } that terminates the main() function returns a
  value of 0.