build: prevent stability issues by overridinge time_t to 32bit on Windows

`-D_FILE_OFFSET_BITS=64` also changes the size of time_t to 64 bits. This is a bug in mingw: http://sourceforge.net/p/mingw-w64/bugs/279/

Override this by adding `-D_USE_32BIT_TIME_T`.

According to @theuni: 'sizeof(time_t) should be 4 for win32. If it's 8, the headers are buggy and runtime will be unpredictable. At runtime, a 4byte value is stored in an 8byte var, leaving half uninit'd. Using a recent mingw it's 4 as expected. I'm assuming gitian's version still has this bug.'