cmd/go: include source dir in cache key when ${SRCDIR} is present in cgo header

[josharian]

What I did:

• have a project including cgo headers using ${SRCDIR}
• move that project to a new directory
• build

Result:

ld: warning: directory not found for option '-L/Users/josh/...redacted...

The redacted directory is the previous directory pointed to by the ${SRCDIR} directive in the cgo sources.

After a go clean -cache, the error disappeared.

I suspect that cmd/go's caching may be too effective here. It probably doesn't take into account the absolute path containing the project, but it may need to when the ${SRCDIR} cgo directive is present.

cc @bcmills @jayconrod

[jayconrod]

When this is fixed, we should also test with -trimpath. When -trimpath is passed, most paths are not included in the cache key, but I think ${SRCDIR} needs to be there in any case.