[patch][pr12684] Use the new mingw abi

[Rafael Espíndola]

Now that 3.4 branched and we will soon require gcc 4.7 to build, it
probably makes more sense to use the new gcc 4.7 abi on mingw.

The difference is documented is
http://gcc.gnu.org/gcc-4.7/changes.html. GCC now uses thiscall by
default for methods. This also has the benefit of helping shake down
bugs since this is also true in the win32 abi.

Cheers,
Rafael
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