The purpose of these headers is to fix issues with mingw v4.0, as described in #9246.

This works by adding this directory to GCC include search path before mingw system headers directories,
so we can intercept their inclusions and add missing definitions without having to modify files in mingw/include.
Disabled tests which now fail on Windows+mingw4.0 due to GCC 4.8 ABI change (#9205).
These really should have been marked xfail-win32, but that doesn't exist, so xfail-fast it is.
auto merge of #9254 : vadimcn/rust/fix-mingw-v4, r=brson

Fix Rust build on mingw v4.0
See #9246 for details.