[GTK] basic OpenWebRTC build support

[GTK] basic OpenWebRTC build support
https://bugs.webkit.org/show_bug.cgi?id=142393

Reviewed by Carlos Garcia Campos.

.:

* Source/cmake/FindOpenWebRTC.cmake: Added.
* Source/cmake/OptionsGTK.cmake: Look for OpenWebRTC library if
mediastream build is enabled.

Source/WebCore:

* PlatformGTK.cmake: Add OpenWebRTC CFLAGS and LDFLAGS.

Tools:

* gtk/jhbuild.modules: Add OpenWebRTC and its dependencies. This
is not the upstream OpenWebRTC repository. This temporary fork
contains a few patches needed by WebKit for audio/video rendering.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@181624 268f45cc-cd09-0410-ab3c-d52691b4dbfc