Measuring time accurately on Linux

It's my first time posting to users mailing list so hello everyone.

I'm currently optimizing code of my server written in Swift. To do that reliably I want to be able to measure how long it takes to execute specific methods.

On OSX I'm using mach_absolute_time() that I've wrapped in a struct for easier interaction and it works fine. Now I'd like to do the same tests on a copy of my actual platform that runs Ubuntu 15.10.

Mach0 library that I use is not available on Linux, instead I could use gettimeofday or clock_gettime. For that I would have to expose time.h to my swift code, however I'm unsure how to do it exactly.
I have started putting my timing struct in a swift package as a first step. Now I'm stuck and here are my questions:
* How exactly do I use clang's modulemap with package manager?
* How do I make modulemap include time.h only on Linux?
* Is it necessary to include full path to time.h header or I can omit it in case of system headers?

Here is a link to my package for future reference (no linux code there for now).

Also, if anyone is aware of a package that uses C library in place of OSX C method to work cross-platform please let me know. It would be a great guidance for me as I have a very vague idea of how package manager and C libs import work.

M. Kalinowski