Windows: Implement setitimer() and sigaction().

The timer is implemented using a thread that calls the signal handler
at regular intervals.

We also replace Windows's signal() function because we must intercept
that SIGALRM is set (which is used when a timer is canceled).

Signed-off-by: Johannes Sixt <johannes.sixt@telecom.at>

compat/mingw.h

@@ -154,6 +154,9 @@ char *mingw_getcwd(char *pointer, int len);
 int mingw_rename(const char*, const char*);
 #define rename mingw_rename
 
+sig_handler_t mingw_signal(int sig, sig_handler_t handler);
+#define signal mingw_signal
+
 /*
  * git specific compatibility
  */