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>

git-compat-util.h

@@ -63,17 +63,18 @@
 #include <sys/time.h>
 #include <time.h>
 #include <signal.h>
-#include <sys/wait.h>
 #include <fnmatch.h>
+#include <assert.h>
+#include <regex.h>
+#include <utime.h>
+#ifndef __MINGW32__
+#include <sys/wait.h>
 #include <sys/poll.h>
 #include <sys/socket.h>
 #include <sys/ioctl.h>
-#include <utime.h>
 #ifndef NO_SYS_SELECT_H
 #include <sys/select.h>
 #endif
-#include <assert.h>
-#include <regex.h>
 #include <netinet/in.h>
 #include <netinet/tcp.h>
 #include <arpa/inet.h>
@@ -89,6 +90,10 @@
 #include <grp.h>
 #define _ALL_SOURCE 1
 #endif
+#else 	/* __MINGW32__ */
+/* pull in Windows compatibility stuff */
+#include "compat/mingw.h"
+#endif	/* __MINGW32__ */
 
 #ifndef NO_ICONV
 #include <iconv.h>