cmd/cgo: record CGO_LDFLAGS env var in _cgo_flags

cgo stores cgo LDFLAGS in _cgo_flags and _cgo_defun.c.
The _cgo_defun.c records the flags via
"#pragma cgo_ldflag <flag>", which external linking
relies upon for passing libraries (and search paths)
to the host linker.

The go command will allow LDFLAGS for cgo to be passed
through the environment (CGO_LDFLAGS); cgo ignores
this environment variable, and so its value doesn't
make it into the above mentioned files. This CL changes
cgo to record CGO_LDFLAGS also.

Fixes #5205.

R=iant, minux.ma
CC=golang-dev
https://golang.org/cl/8465043

src/cmd/cgo/main.go

@@ -208,6 +208,15 @@ func main() {
 
 	p := newPackage(args[:i])
 
+	// Record CGO_LDFLAGS from the environment for external linking.
+	if ldflags := os.Getenv("CGO_LDFLAGS"); ldflags != "" {
+		args, err := splitQuoted(ldflags)
+		if err != nil {
+			fatalf("bad CGO_LDFLAGS: %q (%s)", ldflags, err)
+		}
+		p.addToFlag("LDFLAGS", args)
+	}
+
 	// Need a unique prefix for the global C symbols that
 	// we use to coordinate between gcc and ourselves.
 	// We already put _cgo_ at the beginning, so the main