Libervia: New setup.py compatible with archlinux PKGBUILD. This file define a new variable: ENV_LIBERVIA_INSTALL which is equal to "arch" when libervia is installed
with PKGBUILD. This setup.py solves the following issues: * os.symlink(os.path.dirname(libervia.__file__), os.path.join(tmp_dir,"libervia")) fails with PKGBUILD because
the module libervia is not accessible. * self.custom_create_links() fails with PKGBUILD because the permissions of bin directory is read-only.