make build pick up cached packages only when compiler flags match

This looks related to the discussion in issue #14319, in particular this comment about making -N part of the build signature:

There's also the question of what should happen if you mix packages compiled with and without -N and whether -N should be part of the build signature. I think it's safe to mix packages as long as there are no "go:nosplit" functions in the user packages (auto-nosplit should be fine, since their bound is small and fixed) and user code can't directly call nosplit runtime functions with large frames (which I think is true). That's a lot of "if"s, and it's also strange that the same build command can result in different binaries depending on which packages are clean. To me, this indicates -N should be part of the build signature. I'm not sure if we do that for other flags.

Looks like the discussion in the old thread stalled, though. (And that issue is in the unplanned milestone)

That comment wouldn't fully solve the problem however, even if build didn't mix packages built with and without -N it still couldn't build runtime with -N.

Related (possible dup?): #18369

Work on better caching will fix this. I have a prototype. Not for Go 1.9 but probably for Go 1.10.

Great, thank you. I should mention that last week I've discovered that on tip now I can build with both -a and -gcflags="-N -l" together, which provides a workaround for this problem.

Change https://golang.org/cl/73212 mentions this issue: cmd/go: switch to entirely content-based staleness determination