[HOTFIX] Fix Mac standalone build

Currently the standalone build doesn't work on the bots, so it tends to
break without warning.

This patch fixes an issue where a static member variable that has a
definition but not a separate declaration causes a linking error on Mac
OS X. Not sure why it doesn't happen on other platforms.

For more information on this issue, see:

https://stackoverflow.com/questions/8016780/undefined-reference-to-static-constexpr-char

TBR=miu@chromium.org

Change-Id: I97a7624ef77f85ed0995052d6db5e28834a88dff
Reviewed-on: https://chromium-review.googlesource.com/c/openscreen/+/2568900
Reviewed-by: Jordan Bayles <jophba@chromium.org>
diff --git a/cast/standalone_sender/receiver_chooser.cc b/cast/standalone_sender/receiver_chooser.cc
index 7d6732e..828ea8e 100644
--- a/cast/standalone_sender/receiver_chooser.cc
+++ b/cast/standalone_sender/receiver_chooser.cc
@@ -16,6 +16,12 @@
 namespace openscreen {
 namespace cast {
 
+// NOTE: the compile requires a definition as well as the declaration
+// in the header.
+// TODO(issuetracker.google.com/174081818): move to inline C++17 feature.
+constexpr decltype(ReceiverChooser::kWaitForStragglersDelay)
+    ReceiverChooser::kWaitForStragglersDelay;
+
 ReceiverChooser::ReceiverChooser(const InterfaceInfo& interface,
                                  TaskRunner* task_runner,
                                  ResultCallback result_callback)