Add target architecture MinGW.

With this change GIT can be compiled and linked using MinGW. Builtins
that only read the repository such as the log family and grep already
work.

Simple stubs are provided for a number of functions that the Windows C
runtime does not offer. They will be completed in later patches.
However, a fix for the snprintf/vsnprintf replacement is applied here
to avoid buffer overflows.

Dmitry Kakurin pointed out that access(..., X_OK) would always fails on
Vista and suggested the -D__USE_MINGW_ACCESS workaround.

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

compat/mingw.c

@@ -0,0 +1,57 @@
+#include "../git-compat-util.h"
+
+unsigned int _CRT_fmode = _O_BINARY;
+
+unsigned int sleep (unsigned int seconds)
+{
+	Sleep(seconds*1000);
+	return 0;
+}
+
+int mkstemp(char *template)
+{
+	char *filename = mktemp(template);
+	if (filename == NULL)
+		return -1;
+	return open(filename, O_RDWR | O_CREAT, 0600);
+}
+
+int gettimeofday(struct timeval *tv, void *tz)
+{
+	return -1;
+}
+
+int poll(struct pollfd *ufds, unsigned int nfds, int timeout)
+{
+	return -1;
+}
+
+struct tm *gmtime_r(const time_t *timep, struct tm *result)
+{
+	/* gmtime() in MSVCRT.DLL is thread-safe, but not reentrant */
+	memcpy(result, gmtime(timep), sizeof(struct tm));
+	return result;
+}
+
+struct tm *localtime_r(const time_t *timep, struct tm *result)
+{
+	/* localtime() in MSVCRT.DLL is thread-safe, but not reentrant */
+	memcpy(result, localtime(timep), sizeof(struct tm));
+	return result;
+}
+
+struct passwd *getpwuid(int uid)
+{
+	static struct passwd p;
+	return &p;
+}
+
+int setitimer(int type, struct itimerval *in, struct itimerval *out)
+{
+	return -1;
+}
+
+int sigaction(int sig, struct sigaction *in, struct sigaction *out)
+{
+	return -1;
+}