Split targets mixing .c and .cc sources.

webrtc/modules/audio_device/BUILD.gn

Index: webrtc/modules/audio_device/BUILD.gn
diff --git a/webrtc/modules/audio_device/BUILD.gn b/webrtc/modules/audio_device/BUILD.gn
index 7a0bce0736848319b14941cf60d05eaa678e3ab5..470c9ffb57639b2ab3300db40888c4546866b3ac 100644
--- a/webrtc/modules/audio_device/BUILD.gn
+++ b/webrtc/modules/audio_device/BUILD.gn
@@ -159,10 +159,8 @@ rtc_static_library("audio_device") {
           "mac/audio_device_mac.h",
           "mac/audio_mixer_manager_mac.cc",
           "mac/audio_mixer_manager_mac.h",
-          "mac/portaudio/pa_memorybarrier.h",
-          "mac/portaudio/pa_ringbuffer.c",
-          "mac/portaudio/pa_ringbuffer.h",
         ]
+        deps += [ ":mac_portaudio" ]
         libs = [
           # Needed for CoreGraphics:
           "ApplicationServices.framework",
@@ -243,6 +241,15 @@ rtc_static_library("audio_device") {
   }
 }
 
+rtc_source_set("mac_portaudio") {
+  visibility = [ ":*" ]  # Only targets in this file can depend on this.
+  sources = [
+    "mac/portaudio/pa_memorybarrier.h",
+    "mac/portaudio/pa_ringbuffer.c",
+    "mac/portaudio/pa_ringbuffer.h",
+  ]
+}
+
 config("mock_audio_device_config") {
   if (is_win) {
     cflags = [