making it possible to build polipo using MinGW-w64

fixing this error
`util.o: In function `mkgmtime':
/usr/i686-w64-mingw32/sys-root/mingw/include/time.h:241: undefined reference to `_mkgmtime32'
collect2: error: ld returned 1 exit status
Makefile:76: recipe for target 'polipo.exe' failed
make: *** [polipo.exe] Error 1`