Fall back to fast_float when C++ stdlib doesn't provide from_chars for floats

Hello all,

On deskutils/treesheets port, upstream did some updated that broke build.
After investigating it upstream says we are building with an outdated C++
standard lib.

To fix it, I'm using a fall back to fast_float as we see in main PR:

https://github.com/aardappel/treesheets/issues/686

As I don't have skills to understand why this fix works, I'd like to have
some expert opinion on this fix since I don't know if this will be
supported in the furure.

Resuming we added __FreeBSD__ to fall back in code already present:

<snip>
template<typename T> T parse_float(string_view sv, const char **end =
nullptr) {
    // FIXME: Upgrade compilers for these platforms on CI.
-    #if defined(__APPLE__) || defined(__ANDROID__) ||
defined(__EMSCRIPTEN__)
+    #if defined(__APPLE__) || defined(__ANDROID__) ||
defined(__EMSCRIPTEN__) || defined(__FreeBSD__)
        auto &term = *(char *)(sv.data() + sv.size());
        auto orig = term;
        term = 0;
        auto v = (T)strtod(sv.data(), (char **)end);
        term = orig;
        return v;
    #else
        T val = 0;
        auto res = from_chars(sv.data(), sv.data() + sv.size(), val);
        if (end) *end = res.ptr;
        return val;
    #endif
}
<snip>

Commit is
https://cgit.freebsd.org/ports/commit/?id=4ffd449f0cfdbef7d1c78442ad3ea0be9ab12ea3

Thanks,

-- 
Nuno Teixeira
FreeBSD UNIX:  <eduardo@FreeBSD.org>   Web:  https://FreeBSD.org