Make transient suppression optionally excludable via defines

This allows clients to exclude the transient suppression submodule from WebRTC builds, by defining WEBRTC_EXCLUDE_TRANSIENT_SUPPRESSOR.

The changes have been shown to be bitexact for a test dataset (when the flag is _not_ defined.)

No-Try: True
Bug: webrtc:11226, webrtc:11292
Change-Id: I6931c82a280a9b40a53ee1c2a9820ed9e674a9a5
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/171421
Commit-Queue: Sam Zackrisson <saza@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Per Ã…hgren <peah@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#30978}
diff --git a/BUILD.gn b/BUILD.gn
index d56f549..b3e7710 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -277,6 +277,10 @@
     defines += [ "RTC_DISABLE_METRICS" ]
   }
 
+  if (rtc_exclude_transient_suppressor) {
+    defines += [ "WEBRTC_EXCLUDE_TRANSIENT_SUPPRESSOR" ]
+  }
+
   cflags = []
 
   if (build_with_chromium) {