double-conversion: update to 3.2.1.

What's Changed

    Disable quiet nan test on windows by @floitsch in #174
    Test on all platforms. by @floitsch in #175
    Fix warnings on Windows by @floitsch in #176
    Run ctests first. by @floitsch in #177
    Give shared-lib option and test install by @floitsch in #179
    Install Windows debbuger (pdb) files. by @floitsch in #178
    Add a cast to silence a signedness conversion warning. by @pkasting in #182
    Issue #184 : Fixed all -Wzero-as-null-pointer-constant warnings by @seanm in #185

(wiz)