MinGW: simplify waitpid() emulation macros

Windows does not have signals. At least they cannot be diagnosed by the
parent process; all that the parent process can observe is the exit code.

This also adds a dummy definition of WTERMSIG.
Signed-off-by: NJohannes Sixt <j6t@kdbg.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

compat/mingw.h

@@ -17,9 +17,10 @@ typedef int pid_t;
 #define S_IROTH 0
 #define S_IXOTH 0
 
-#define WIFEXITED(x) ((unsigned)(x) < 259)	/* STILL_ACTIVE */
+#define WIFEXITED(x) 1
+#define WIFSIGNALED(x) 0
 #define WEXITSTATUS(x) ((x) & 0xff)
-#define WIFSIGNALED(x) ((unsigned)(x) > 259)
+#define WTERMSIG(x) SIGTERM
 
 #define SIGHUP 1
 #define SIGQUIT 3