[Mingw-cross-env-list] C++ 11 Enabled By Default

[Tony Theodore]

Hi All,

From "What to do with libraries which require GCC built with posix threads?”[1]:

> More and more C++ based projects currently begin to use treads implementation 
> from C++11 (<thread>, <mutex>, <future>, etc.).

These features are now enabled by default[2]. There should be no impact on any 
existing scripts, any issues
with `pthread` detection would have occurred when we started building it inline 
with `gcc` in late 2015.

Using the `.win32` suffix on targets will disable these features, but that 
isn’t recommended as we won’t be keeping
up with disabling packages that now require a posix toolchain.

Cheers,

Tony

[1] https://github.com/mxe/mxe/issues/2258
[2] https://github.com/mxe/mxe/pull/2263
[3] https://github.com/mxe/mxe/pull/958