strtod() on Solaris does not support hex strings

For the ESI plugins, we have a regresion test:

    assert(esi_expr.evaluate("$(HTTP_COOKIE{grade}) < -0x2"));

This fails, because on OmniOS / Solaris, g++ does not seem to think it can / should support hex strings. However, strtold() always conforms to C99/SUSv3 on Solaris, so we can use that.