build.go: add an option to specify '-o' option when build

[bekcpear]

When GOBIN is set, 'go install' cannot install cross-compilied binaries.
To satisfy cross-compilation, it's necessary to add the '-o' to build
target, otherwise 'go build' will discarding the resulting objects when
compiling multiple packages.

Purpose

To satisfy cross-compilation.
To satisfy cross-compilation for 'all' target.

[calmh]

But the option you're adding is for the build subcommand, not install, which already handles cross compilation? I mean, it's how we cross compile.

[bekcpear]

But if I run

go run build.go -goarch arm64 install

An error go install: cannot install cross-compiled binaries when GOBIN is set occured, unless unset the GOBIN env.

[calmh]

go run build.go -goarch arm64 build

Builds just the named target, or syncthing by default, to the current directory. Use this when cross compiling, with parameters for what to cross compile to: go run build.go -goos linux -goarch 386 build.

(https://docs.syncthing.net/dev/building.html)

[bekcpear]

Yes, this command is okay, but as I said:

'go build' will discarding the resulting objects when compiling multiple packages.

command

go run build.go -goarch arm64 build all

takes no effect, unless '-o' option specified.

[calmh]

Right, that seems like it could be legit useful