time_r: refactor tm_zone tests * m4/tm_gmtoff.m4 (gl_TM_GMTOFF): Also check for tm_zone and define HAVE_STRUCT_TM_TM_ZONE accordingly. * modules/time_r-tests (Files): Add m4/tm_gmtoff.m4. (configure.ac): Simplify by using gl_TM_GMTOFF.