I am running on MAC OSX El Capitan. My version of postgres is 126.96.36.199. My PostGIS version is 2.1.7. I cannot upgrade to PostGIS 2.2, because when I am trying to do, it pops "PostGIS requires SFCGAL >= 1.1.0 (found 1.0.5)". Than when I try to fix it by upgrading SFCGAL, it stops process in the steps of compiling "union.cpp" library. I have found on github, that the problem might be in the old version of Boost, so I have upgraded Boost, but the problem has preserved.
I should also mention, if you don't need SFCGAL support you should be able to turn off with
functions listed here: http://postgis.net/docs/manual-2.2/reference.html#reference_sfcgal (ST_3DIntersects and some other functions also have different behavior if backend is set to sfcgal)
functions affected you'll see with a 1/2 3D thingy http://postgis.net/docs/manual-2.2/PostGIS_Special_Functions_Index.html#PostGIS_TypeFunctionMatrix (are functions built into PostGIS but are enhanced when backend=sfcgal or just behave differently).