Return environment dict.

        Parameters
        ----------
        exists: bool
            It True, only return existing paths.

def return_env(self, exists=True):
        """
        Return environment dict.

        Parameters
        ----------
        exists: bool
            It True, only return existing paths.
        """
        env = dict(
            include=self._build_paths('include',
                                      [self.VCIncludes,
                                       self.OSIncludes,
                                       self.UCRTIncludes,
                                       self.NetFxSDKIncludes],
                                      exists),
            lib=self._build_paths('lib',
                                  [self.VCLibraries,
                                   self.OSLibraries,
                                   self.FxTools,
                                   self.UCRTLibraries,
                                   self.NetFxSDKLibraries],
                                  exists),
            libpath=self._build_paths('libpath',
                                      [self.VCLibraries,
                                       self.FxTools,
                                       self.VCStoreRefs,
                                       self.OSLibpath],
                                      exists),
            path=self._build_paths('path',
                                   [self.VCTools,
                                    self.VSTools,
                                    self.VsTDb,
                                    self.SdkTools,
                                    self.SdkSetup,
                                    self.FxTools,
                                    self.MSBuild,
                                    self.HTMLHelpWorkshop,
                                    self.FSharp],
                                   exists),
        )
        if self.vc_ver >= 14 and os.path.isfile(self.VCRuntimeRedist):
            env['py_vcruntime_redist'] = self.VCRuntimeRedist
        return env