cmd/go: disable warnings from cmd/cc when building for SWIG

Fixes #9065.

LGTM=rsc
R=rsc, misch
CC=golang-codereviews
https://golang.org/cl/171270043

src/cmd/go/build.go

@@ -1826,7 +1826,15 @@ func (gcToolchain) ld(b *builder, p *Package, out string, allactions []*action,
 func (gcToolchain) cc(b *builder, p *Package, objdir, ofile, cfile string) error {
 	inc := filepath.Join(goroot, "pkg", fmt.Sprintf("%s_%s", goos, goarch))
 	cfile = mkAbs(p.Dir, cfile)
-	args := stringList(tool(archChar+"c"), "-F", "-V", "-w", "-trimpath", b.work, "-I", objdir, "-I", inc, "-o", ofile, buildCcflags, "-D", "GOOS_"+goos, "-D", "GOARCH_"+goarch, cfile)
+	warn := []string{"-w"}
+	if p.usesSwig() {
+		// When using SWIG, this compiler is only used to
+		// compile the C files generated by SWIG.
+		// We don't want warnings.
+		// See issue 9065 for details.
+		warn = nil
+	}
+	args := stringList(tool(archChar+"c"), "-F", "-V", warn, "-trimpath", b.work, "-I", objdir, "-I", inc, "-o", ofile, buildCcflags, "-D", "GOOS_"+goos, "-D", "GOARCH_"+goarch, cfile)
 	return b.run(p.Dir, p.ImportPath, nil, args)
 }