message: Use gnu11 standard when none is specified
  Reference:
  1.
  2.
diff: @@ -137,7 +137,7 @@ Project::Entry GetCompilationEntryFromCompileCommandEntry(
  }
  if (!AnyStartsWith(entry.args, "-std=")) {
  if (*source_file_type == "c")
  - result.args.push_back("-std=c11");
  + result.args.push_back("-std=gnu11");
  else if (*source_file_type == "c++")
  result.args.push_back("-std=c++11");
  }
  @@ -516,7 +516,7 @@ TEST_SUITE("Project") {
  "/home/user", "/home/user/foo/bar.c",
  /* raw */ {"cc", "-O0", "foo/bar.c"},
  /* expected */
  - {"cc", "-working-directory", "/home/user", "-xc", "-std=c11", "-O0", "-resource-dir=/w/resource_dir/",
  + {"cc", "-working-directory", "/home/user", "-xc", "-std=gnu11", "-O0", "-resource-dir=/w/resource_dir/",
  "-Wno-unknown-warning-option", "-fparse-all-comments"});
  }
  