copysignl: Simplify for platforms where 'long double' == 'double'. * lib/copysignl.c (copysignl) [HAVE_SAME_LONG_DOUBLE_AS_DOUBLE]: New alternative. * m4/copysignl.m4 (gl_FUNC_COPYSIGNL): Require gl_LONG_DOUBLE_VS_DOUBLE. Determine COPYSIGNL_LIBM according to HAVE_SAME_LONG_DOUBLE_AS_DOUBLE. * modules/copysignl (Depends-on): Add copysign. Update conditions.