cgo: don't run cgo when not compiling
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
http://codereview.appspot.com/4023069

Committer: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang