Disable -Werror=maybe-uninitialized as JavaScriptCore doesn't compile on g++ 4.8.1.

This flag also caused errors (already fixed) in WebCore, to fix those errors usually is
needed to add useless code that reduce the readability in favor of let the compiler
happy.

References:
https://bugs.webkit.org/show_bug.cgi?id=118534
https://bugs.webkit.org/show_bug.cgi?id=116340

Reviewed by: Sergio Correia