how to build opencv_contrib/sfm for ros kinetic?

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I am on ubuntu with ros kinetic and Opencv3-2 by default, and I need to use a pakcage that's located in opencv_contrib

I noticed that some packages of it already in my system (with Ros), like xfeatures.SIFT, the wiki however is unclear about this .

Somebody my say : rebuild everthing (opencv and opencv_contrib) , but again how?, and where is opencv build folder located? though this is diffuculte choice for me