strtod (and atof) of "5.30498947741318e-315" returns 5.3049894774131808e-315 but
with errno set to ERANGE. This is new in 2.10.1 compared to 2.9 and does not
appear to be correct.
That's the correct behavior.
I guess it depends on what you think "cannot be represented, without
extraordinary roundoff error" (definition of underflow from the C99 standard
[126.96.36.199]) means. 5.3049894774131808e-315 compared to 5.30498947741318e-315
does not appear to me to have "extraordinary roundoff error". I would suggest
that something other than a simple comparison with 2.2250738585072014E-308 for
triggering ERANGE is in order.