Linux: add a tool and code to make use_system_ffmpeg option more compatible

This introduces a compile-time detection of ffmpeg configuration
(which codecs are available).

See https://groups.google.com/a/chromium.org/d/msg/chromium-dev/fm5Oe_AC3Sc/qkbmC7txaSkJ
for more context.

No functional change for Google Chrome.

BUG=none

Review URL: https://codereview.chromium.org/12302029

git-svn-id: http://src.chromium.org/svn/trunk/src/build@183463 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

common.gypi

@@ -741,6 +741,9 @@
     'google_default_client_secret%': '<(google_default_client_secret)',
     'enable_managed_users%': '<(enable_managed_users)',
 
+    # Use system ffmpeg instead of bundled one.
+    'use_system_ffmpeg%': 0,
+
     # Use system mesa instead of bundled one.
     'use_system_mesa%': 0,