Does not compile under Clang 3.7

[dmcconachie]

Switching to clang briefly generates the following error:

catkin_ws/src/arc_utilities/include/arc_utilities/arc_helpers.hpp:17:10: fatal error: 'omp.h' file not found
#include <omp.h>
         ^
1 error generated.

Presumably this is due to missing compile flags:
https://stackoverflow.com/questions/33400462/omp-h-file-not-found-when-compiling-using-clang
https://cmake.org/cmake/help/v3.0/module/FindOpenMP.html

We also likely do not want to set these flags across the whole library, instead leaving them to be set by each end user library. We ought to find a way to let these headers be used under clang with or without OpenMP.