The logic introduced to avoid running cgo when
introducing _cgo_flags is faulty.  My goal was
to handle it with the following statement:

    -include _cgo_flags

The dash tells make to ignore errors if it can't
include the file.

What I missed, though, was the fact that it
*will* attempt to build the file if it knows
how.

This change will introduce the originally
intended semantics of not attempting to build
the file before necessary.

R=rsc
CC=golang-dev
https://golang.org/cl/4023069