Fix //tools/gn/example on Linux: missing current_os and related vars

tools/gn/example/build/BUILDCONFIG.gn

Index: tools/gn/example/build/BUILDCONFIG.gn
diff --git a/tools/gn/example/build/BUILDCONFIG.gn b/tools/gn/example/build/BUILDCONFIG.gn
index d107908ab521d900ed8fd5157b046445a7743123..e419fd96a8a4c769849ce45839c8d8cd71eadf43 100644
--- a/tools/gn/example/build/BUILDCONFIG.gn
+++ b/tools/gn/example/build/BUILDCONFIG.gn
@@ -2,6 +2,19 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+if (target_os == "") {
+  target_os = host_os
+}
+if (target_cpu == "") {
+  target_cpu = host_cpu
+}
+if (current_cpu == "") {
+  current_cpu = target_cpu
+}
+if (current_os == "") {
+  current_os = target_os
+}
+
 # All binary targets will get this list of configs by default.
 _shared_binary_target_configs = [ "//build:compiler_defaults" ]