Fixed compile error in MinGW (#260)

* Fixed compile error in MinGW

Some versions of MinGW miss some definitions. In order to be able to
compile I redefined a function and added an explicit include. It is only
a quick workaround, as other people also seemed to have the same
problem. It should be checked whether a better solution is possible.

* Removed _ftime_s redefine, calling _ftime directly