cmd/dlv: do not override user's CGO_CFLAGS

Fixes #1278

cmd/dlv/main.go

@@ -5,6 +5,7 @@ import (
 
 	"github.com/derekparker/delve/cmd/dlv/cmds"
 	"github.com/derekparker/delve/pkg/version"
+	"github.com/sirupsen/logrus"
 )
 
 // Build is the git sha of this binaries build.
@@ -14,6 +15,11 @@ func main() {
 	if Build != "" {
 		version.DelveVersion.Build = Build
 	}
-	os.Setenv("CGO_CFLAGS", "-O -g")
+	const cgoCflagsEnv = "CGO_CFLAGS"
+	if os.Getenv(cgoCflagsEnv) == "" {
+		os.Setenv(cgoCflagsEnv, "-O -g")
+	} else {
+		logrus.WithFields(logrus.Fields{"layer": "dlv"}).Warnln("CGO_CFLAGS already set, Cgo code could be optimized.")
+	}
 	cmds.New(false).Execute()
 }