use MinGW-w64 v3.1.0 for TBB 4.x
As discussed during the 13-Jun-2014 TBB dev meeting, I propose that we upgrade to MinGW-w64 v3.1.0 (latest stable version) for the TBB 4.x series. Doing so resolves some build issues with the Firefox updater code and will (hopefully) make things easier when we start to use ESR 31 code.
The newer MinGW also requires newer versions of gcc and binutils. I did a test build with gcc 4.8.3 (latest stable 4.8.x version) and binutils 2.24 (latest stable version). I did not notice any obvious problems.
brade and I will make our build changes available on a branch so that Mike and/or GK can review.
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