strtof, strtod, strtold: Fix underflow behaviour of system function.
* m4/strtof.m4 (gl_FUNC_STRTOF): Test for strtof's behaviour upon
underflow. Conditionally define STRTOF_HAS_UNDERFLOW_BUG,
STRTOF_HAS_GRADUAL_UNDERFLOW_PROBLEM.
* m4/strtod.m4 (gl_FUNC_STRTOD): Test for strtod's behaviour upon
underflow. Conditionally define STRTOD_HAS_UNDERFLOW_BUG,
STRTOD_HAS_GRADUAL_UNDERFLOW_PROBLEM.
* m4/strtold.m4 (gl_FUNC_STRTOLD): Test for strtold's behaviour upon
gradual underflow. Conditionally define
STRTOLD_HAS_GRADUAL_UNDERFLOW_PROBLEM.
* lib/strtod.c (HAVE_UNDERLYING_STRTOD): Test STRTOF_HAS_UNDERFLOW_BUG,
STRTOD_HAS_UNDERFLOW_BUG.
(HAS_GRADUAL_UNDERFLOW_PROBLEM): New macro.
(SET_ERRNO_UPON_GRADUAL_UNDERFLOW): New macro.
(STRTOD): Use it.