The MinGW-w64 version we have on our Travis build environment (Ubuntu 12.04,
mingw-w64 2.0.1, gcc 4.6) is old and has some missing includes in the
dependencies of the `tcpmib.h` header [0] [1] [2].
Those were not triggered before 6323779596
probably due to conflicting WINVER definitions which prevented triggering the code
specific to >= 0x0600 (Vista). We ensure it won't be triggered by defining the
_WIN32_WINNT macro to Windows XP compatibility.
b24fe6879a