cgo: don't run cgo when not compiling
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