Add const in BearingAngleImage::getAngle

cf.
http://www.pcl-users.org/Problem-building-from-trunk-pcl-commons-tp4031835.html.
Fixes Mac OSX 10.9 with libc++

Only switches to libstdc++ when you actually have CUDA and you enabled it.
Merge pull request #487 from fran6co/patch-7

Fixes Mac OSX 10.9 with libc++
Merge pull request #481 from jspricke/constfix

Add const in BearingAngleImage::getAngle
Merge branch 'master' of https://github.com/PointCloudLibrary/pcl