def _set_default_properties_from_env(self) -> None:
        """Properties which can also be set from the environment."""
        # name, evar, split
        opts: T.List[T.Tuple[str, T.List[str], bool]] = [
            ('boost_includedir', ['BOOST_INCLUDEDIR'], False),
            ('boost_librarydir', ['BOOST_LIBRARYDIR'], False),
            ('boost_root', ['BOOST_ROOT', 'BOOSTROOT'], True),
            ('java_home', ['JAVA_HOME'], False),
        ]

        for (name, evars, split), for_machine in itertools.product(opts, MachineChoice):
            for evar in evars:
                p_env = _get_env_var(for_machine, self.is_cross_build(), evar)
                if p_env is not None:
                    if split:
                        self.properties[for_machine].properties.setdefault(name, p_env.split(os.pathsep))
                    else:
                        self.properties[for_machine].properties.setdefault(name, p_env)
                    break
Properties which can also be set from the environment.
mesonbuild/environment.py