pkgconf_tuple_find
var = pkgconf_tuple_find(client, &pkg->vars, variable);
PKGCONF_API char *pkgconf_tuple_find(const pkgconf_client_t *client, pkgconf_list_t *list, const char *key);
char *pkg_sysroot_dir = pkgconf_tuple_find(client, &pkg->vars, "pc_sysrootdir");
sysroot_dir = pkgconf_tuple_find(client, vars, "pc_sysrootdir");
kv = pkgconf_tuple_find(client, vars, varname);