GYP variables for building expat, icu, libsrtp, usrsctp

This makes the build more flexible when linking against
prebuilt external libraries.

Use existing build_* variables for libyuv and json in talk/
(already in use in webrtc/).

Also make it possible to avoid building the GTK parts of the Linux build.

BUG=4242
R=andrew@webrtc.org, tommi@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/44179005

Cr-Commit-Position: refs/heads/master@{#9087}

talk/libjingle_examples.gyp

@@ -97,11 +97,15 @@
             'examples/peerconnection/client/peer_connection_client.h',
           ],
           'dependencies': [
-            '<(DEPTH)/third_party/jsoncpp/jsoncpp.gyp:jsoncpp',
             'libjingle.gyp:libjingle_peerconnection',
             '<@(libjingle_tests_additional_deps)',
           ],
           'conditions': [
+            ['build_json==1', {
+              'dependencies': [
+                '<(DEPTH)/third_party/jsoncpp/jsoncpp.gyp:jsoncpp',
+              ],
+            }],
             # TODO(ronghuawu): Move these files to a win/ directory then they
             # can be excluded automatically.
             ['OS=="win"', {