x/mobile: -tags=ios is not getting set

[prep]

I've been experimenting with the new iOS support in x/mobile and I noticed this compile error when including net/http in mypackage:

$ gomobile bind -target ios mypackage

clang: error: invalid argument '-mmacosx-version-min=10.6' not allowed with '-mios-simulator-version-min=6.1'

The -mios-simulator-version-min=6.1 parameter in x/mobile/cmd/gomobile/env.go seems to conflicts with the -mmacosx-version-min=10.6 CFLAGS parameter in crypto/x509/root_cgo_darwin.go. A +build constraint with an !ios tag seems to be present in the latter file, but it doesn't appear to get honored.

Adding the -tags=ios parameter to my previous gomobile command seems to work, but it would be great if it could be added by default to x/mobile/cmd/gomobile/build.go.

[gopherbot]

CL https://golang.org/cl/12387 mentions this issue.