Make the generated stubs weak. Enable the unittests that had conflicting symbols.

third_party/ffmpeg/generate_stubs.py

Index: third_party/ffmpeg/generate_stubs.py
diff --git a/third_party/ffmpeg/generate_stubs.py b/third_party/ffmpeg/generate_stubs.py
index 5e3715719a67e7a95796f98635ada093611472e7..948ed4ab5b26f7b712d35a8b7fbd44e064695802 100755
--- a/third_party/ffmpeg/generate_stubs.py
+++ b/third_party/ffmpeg/generate_stubs.py
@@ -377,6 +377,10 @@ class PosixStubWriter(object):
   def StubFunction(cls, signature):
     """Generates a stub function definition for the given signature.
 
+    The function definitions are created with __attribute__((weak)) so that
+    they may be overridden by a real static link or mock versions to be used
+    when testing.
+
     Args:
       signature: The hash representing the function signature.
 
@@ -395,7 +399,8 @@ class PosixStubWriter(object):
     if arg_list == 'void':
       arg_list = ''
 
-    return """%(return_type)s %(name)s(%(params)s) {
+    return """extern %(return_type)s %(name)s(%(params)s) __attribute__((weak));
+%(return_type)s %(name)s(%(params)s) {
   %(return_prefix)s%(name)s_ptr(%(arg_list)s);
 }""" % {'return_type': signature['return_type'],
         'name': signature['name'],