FindOPENMP changed since CMake 3.9 (older version breaks SIRFReg)

[KrisThielemans]

Since CMake 3.9, FindOPENMP has OpenMP_<lang>_FOUND and defines the OpenMP::OpenMP_<lang> target. Before that, it had OPENMP_FOUND and OpenMP_CXX_FLAGS (and no link flag, causing a bug on some systems for linking).

This breaks our lines to accomodate OpenMP and NiftyReg

As we'll have boost problem dependencies etc with older CMake as well, I suggest we up to requirements to CMake 3.9. Objections?

[KrisThielemans]

@rijobro, only thing that needs to happen is up requirement for CMake version to 3.9.0 (and therefore edit installation instructions and CHANGES.md)

[rijobro]

Ok

[KrisThielemans]

fixed by #331