Fix thunderbird build with clang 3.4

[Niclas Zeising]

Hi!
Attached patch fixes, or at least makes the build progress longer,
thunderbird when compiled with the upcoming clang 3.4.
See https://bugzilla.mozilla.org/show_bug.cgi?id=894240 for details.
It's needed both in the dev repo and the ports repo.
Regards!
-- 
Niclas Zeising
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