Define BOOST_VARIANT_USE_RELAXED_GET_BY_DEFAULT in case of boost 1.58.0 build
Created by: sanak
Detail description is here (https://github.com/Oslandia/SFCGAL/issues/103#issuecomment-132210048).
I confirmed that both of build and standalone-regress-test-SFCGAL
passed on my Mac OSX Yosemite homebrew environment.