- Fix compilation on OSX using clang-3.8

[rperrot]

Here is a simple fix used to enhance compilation on compliant platforms.

There are three fixes :

• the first one is to force using your cmake/FindEigen instead of the system one ;
• the second one is to use the -std=c++11 as a cmake macro (this enhance portability)
• the third ones are just two missing includes.

Note : I didn't fix one thing : the include <omp.h> in main.cpp. If the target doesn't have openmp (which is the default on clang shipped with Xcode), this cause a compilation error. You may indicate to the user that you require openmp or use #ifdef/#define to prevent missing include.

Hope it helps.