Haiku fixes

[ypsvlq]

various low-hanging fruit, builds usable stage3

see #7691 (comment)

[alexrp]

This looks wrong? Did you mean to return an error?

[ypsvlq]

Currently build_runner calls this unconditionally and other uses are gated behind have_impl, it would probably be better to rework the logic though.

[alexrp]

As far as I can tell, returning an error will get the correct behavior in the build runner.

[ypsvlq]

It should hit this line if have_impl is false:

zig/lib/compiler/build_runner.zig

Line 470 in c907866

if (!Watch.have_impl) fatal("--watch not yet implemented for {s}", .{@tagName(builtin.os.tag)});

[alexrp]

We know upfront whether --watch support is implemented, so checking it in the rebuild loop seems a bit silly. I suggest removing that line and just returning something like error.WatchUnsupportedOnHostOs from Watch.init(), so that this line will bail out early with an error:

zig/lib/compiler/build_runner.zig

Line 418 in c907866

var w = if (watch) try Watch.init() else undefined;

[ypsvlq]

Sure, I tried to leave any behaviour changes out of this PR but can add it here if you want.

[ypsvlq]

Returning an error would look something like e80e96a (or adding a conditional around the watch loop), which I don't think is an improvement.

Returning undefined was not good though, is 84013e1 okay?

[ypsvlq]

helps #21094