Add layering_check to Bazel CI.

This will avoid regressions on #596.

.bazelci/presubmit.yml

@@ -1,25 +1,41 @@
 ---
 platforms:
   ubuntu1604:
+    build_flags:
+    - "--features=layering_check"
     build_targets:
     - "..."
+    test_flags:
+    - "--features=layering_check"
     test_targets:
     - "..."
   ubuntu1804:
+    build_flags:
+    - "--features=layering_check"
     build_targets:
     - "..."
+    test_flags:
+    - "--features=layering_check"
     test_targets:
     - "..."
   macos:
+    build_flags:
+    - "--features=layering_check"
     build_targets:
     - "..."
+    test_flags:
+    - "--features=layering_check"
     test_targets:
     - "..."
   windows:
     # Optional: use VS 2017 instead of 2015.
     environment:
       BAZEL_VC: "C:\\Program Files (x86)\\Microsoft Visual Studio\\2017\\BuildTools\\VC"
+    build_flags:
+    - "--features=layering_check"
     build_targets:
     - "..."
+    test_flags:
+    - "--features=layering_check"
     test_targets:
     - "..."

bazel/example/BUILD.bazel

@@ -4,5 +4,6 @@ cc_test(
     srcs = ["main.cc"],
     deps = [
         "//:glog",
+        "@com_github_gflags_gflags//:gflags",
     ],
 )