I think the problem here is that you’re using too old a version of Qt. As mentioned on the MacOSX installation page, you need at least version 5.1 - preferably the latest available. I think this is what’s throwing off the build script, since for some reason your version of Qt instructs the compiler to target a much older version of MacOSX (-mmacosx-version-min=10.7), when the C++ feature we rely on (std::future) is only available from MacOSX 10.8. There’s a good chance a recent version of Qt would not target such an old version of MacOSX.
If you thought you had indstalled Qt5, then refer to this post for instructions on making sure MRtrix3 builds against it rather than Qt4.