Make rig compilable under OSX.

Also needs to hack .pc files of cogl:
1. cogl-2.0-experimental.pc:
  * Cflags: add '-DCOGL_ENABLE_EXPERIMENTAL_2_0_API'
2. cogl-gst-2.0-experimental.pc:
  * requires: add 'gstreamer-video-1.0'
  * Cflags: add '-DCOGL_ENABLE_EXPERIMENTAL_2_0_API'
3. cogl-pango-2.0-experimental.pc:
  * requires: add 'pango'
  * Libs: '-lcoglpango' should be '-lcogl-pango'
  * Cflags: add '-DCOGL_ENABLE_EXPERIMENTAL_2_0_API'
4. cogl-path-2.0-experimental.pc:
  * Cflags: add '-DCOGL_ENABLE_EXPERIMENTAL_2_0_API'