Make OpenMP available in downstream projects

[mvieth]

Previously, if a downstream project used PCL_NO_PRECOMPILE or if it used a PCL class which includes OpenMP code, but PCL does not make an instantiation of that class, OpenMP was not available. In other words, when PCL code using OpenMP was compiled when building PCL, OpenMP was available. However when PCL code using OpenMP was compiled in the downstream/user project (e.g. in the two mentioned cases), OpenMP was not available. The fix is to search for OpenMP again in PCLConfig.cmake

Fixes #5717

[larshg]

I think we should add:

io:

pcl/io/include/pcl/io/impl/lzf_image_io.hpp

Line 161 in f0c7c46

#pragma omp parallel for \

Segmentation:

pcl/segmentation/include/pcl/segmentation/impl/approximate_progressive_morphological_filter.hpp

Line 157 in f0c7c46

#pragma omp parallel for \

Surface:

pcl/surface/include/pcl/surface/impl/mls.hpp

Line 301 in f0c7c46

#pragma omp parallel for \

Tracking:

pcl/tracking/include/pcl/tracking/impl/particle_filter_omp.hpp

Line 1 in f0c7c46

#ifndef PCL_TRACKING_IMPL_PARTICLE_OMP_FILTER_H_

Or do you assume they will be added due to public linking of OpenMP flags/libraries?

[mvieth]

@larshg Good point, I added them.