Modify mingw_main() workaround to avoid link errors

With MinGW's

   gcc.exe (GCC) 3.4.5 (mingw special)
   GNU ld version 2.17.50 20060824

the old define caused link errors:

   git.o: In function `main':
   C:/msysgit/git/git.c:500: undefined reference to `mingw_main'
   collect2: ld returned 1 exit status

The modified define works.
Signed-off-by: NSteffen Prohaska <prohaska@zib.de>
Acked-by: NJohannes Sixt <johannes.sixt@telecom.at>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

compat/mingw.h

@@ -228,9 +228,10 @@ char **env_setenv(char **env, const char *name);
  * A replacement of main() that ensures that argv[0] has a path
  */
 
-#define main(c,v) main(int argc, const char **argv) \
+#define main(c,v) dummy_decl_mingw_main(); \
+static int mingw_main(); \
+int main(int argc, const char **argv) \
 { \
-	static int mingw_main(); \
 	argv[0] = xstrdup(_pgmptr); \
 	return mingw_main(argc, argv); \
 } \