title: Using select when defining a dict in Bazel
body: <p>In bazel I often see following code:</p>
  
  <pre><code>srcs = [
          "foo/bar.c",
      ] + select({
          "@org_tensorflow//tensorflow:linux_x86_64": [
              "foo/baz.c",
          ],
          "//conditions:default": [],
      })
  </code></pre>
  
  <p>But how do I go with conditionally appending a <code>dict</code> like this?</p>
  
  <pre><code>subs = {
          "#undef HWLOC_VERSION_MAJOR": "#define HWLOC_VERSION_MAJOR 2",
  }
  </code></pre>