[Tar] Add MinGW patches for Windows

[giordano]

I found the MinGW patches for Tar, but it still doesn't quite work:

[00:43:13] system.h:491:21: error: dereferencing pointer to incomplete type
[00:43:13]    if (initgroups (pw->pw_name, getgid ()))
[00:43:13]                      ^
[...]
[00:43:13] rtapelib.c: In function ‘rmt_write__’:
[00:43:13] rtapelib.c:607:26: error: ‘SIGPIPE’ undeclared (first use in this function)
[00:43:13]    pipe_handler = signal (SIGPIPE, SIG_IGN);
[00:43:13]                           ^
[00:43:13] make[3]: *** [Makefile:1502: rtapelib.o] Error 1