Update project to work with Bazel v0.28.0

- Updated to latest version of protobuf, v3.9.0.
- Added `@bazel_skylib` external/workspace dependency. (Seems to be
needed for protobuf.)
- Applied the Bazel v0.28.0 bug workaround described [here](bazelbuild/bazel#8571 (comment)).
(This shouldn't be necessary after the next bazel release.)

.bazelrc

@@ -1,3 +1,3 @@
-build --cxxopt='-std=c++14'
+build --experimental_google_legacy_api --cxxopt='-std=c++14'
 test  --cxxopt='-std=c++14'
 run   --cxxopt='-std=c++14'

WORKSPACE

@@ -2,9 +2,19 @@ workspace(name = "ex_clion_bazel_plugin_gdb_timeout")
 
 load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
 
+http_archive(
+    name = "bazel_skylib",
+    sha256 = "1dde365491125a3db70731e25658dfdd3bc5dbdfd11b840b3e987ecf043c7ca0",
+    url = "https://github.com/bazelbuild/bazel-skylib/releases/download/0.9.0/bazel_skylib-0.9.0.tar.gz",
+)
+
 http_archive(
     name = "com_google_protobuf",
-    sha256 = "9510dd2afc29e7245e9e884336f848c8a6600a14ae726adb6befdb4f786f0be2",
-    strip_prefix = "protobuf-3.6.1.3",
-    urls = ["https://github.com/protocolbuffers/protobuf/archive/v3.6.1.3.zip"],
+    sha256 = "8eb5ca331ab8ca0da2baea7fc0607d86c46c80845deca57109a5d637ccb93bb4",
+    strip_prefix = "protobuf-3.9.0",
+    urls = ["https://github.com/protocolbuffers/protobuf/archive/v3.9.0.zip"],
 )
+
+load("@com_google_protobuf//:protobuf_deps.bzl", "protobuf_deps")
+
+protobuf_deps()