Add rtc_include_alsa GN variable

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 2f678d6dc082529657946346d84dac3e36960508..184d7e1c53726020d186bdf10955ca281f24ef1c 100644
--- a/webrtc/modules/audio_device/BUILD.gn
+++ b/webrtc/modules/audio_device/BUILD.gn
@@ -126,20 +126,22 @@ rtc_static_library("audio_device") {
       defines += [ "WEBRTC_DUMMY_FILE_DEVICES" ]
     } else {
       if (is_linux) {
-        sources += [
-          "linux/alsasymboltable_linux.cc",
-          "linux/alsasymboltable_linux.h",
-          "linux/audio_device_alsa_linux.cc",
-          "linux/audio_device_alsa_linux.h",
-          "linux/audio_mixer_manager_alsa_linux.cc",
-          "linux/audio_mixer_manager_alsa_linux.h",
-          "linux/latebindingsymboltable_linux.cc",
-          "linux/latebindingsymboltable_linux.h",
-        ]
-        defines += [ "LINUX_ALSA" ]
-        libs = [ "dl" ]
-        if (use_x11) {
-          libs += [ "X11" ]
+        if (rtc_include_alsa) {
+          sources += [
+            "linux/alsasymboltable_linux.cc",
+            "linux/alsasymboltable_linux.h",
+            "linux/audio_device_alsa_linux.cc",
+            "linux/audio_device_alsa_linux.h",
+            "linux/audio_mixer_manager_alsa_linux.cc",
+            "linux/audio_mixer_manager_alsa_linux.h",
+            "linux/latebindingsymboltable_linux.cc",
+            "linux/latebindingsymboltable_linux.h",
+          ]
+          defines += [ "LINUX_ALSA" ]
+          libs = [ "dl" ]
+          if (use_x11) {
+            libs += [ "X11" ]
+          }
         }
         if (rtc_include_pulse_audio) {
           sources += [