* src/misc/mtime.c: fixed an overflow in mdate() on win32.

* src/misc/mtime.c: fixed an overflow in mdate() on win32.
  This overflow was only affecting the hardware configurations where we can't use the performance counter and thus have to use GetTickCount().
  We now also check for the wrapping of the 32 bits value returned by GetTickCount() so mdate() doesn't wrap after 49.7 days.
* src/misc/win32_specific.c: call mdate() once in system_Init() so as to avoid thread safety issues while initialising the static variables used in mdate().