MSVC: fix stat definition hell

MSVC: fix stat definition hell

In msvc.h, there's a couple of stat related functions defined diffently
from mingw.h. When we remove these definitions, the only problem we get is
"warning C4005: '_stati64' : macro redefinition" for this line in mingw.h:

#define _stati64(x,y) mingw_stat(x,y)

The reason is that as of MSVCR80.dll (distributed with MSVC 2005), the
original _stati64 family of functions was renamed to _stat32i64, and the
former function names became macros (pointing to the appropriate function
based on the definition of _USE_32BIT_TIME_T).

Defining _stati64 works on MinGW because MinGW by default compiles against
the MSVCRT.DLL that is part of Windows (i.e. _stati64 is a function rather
than a macro).

Note: MinGW *can* compile for newer MSVC runtime versions, and MSVC
apparently can also compile for the Windows MSVCRT.DLL via the DDK (see
http://www.syndicateofideas.com/posts/fighting-the-msvcrt-dll-hell ).

Remove the stat definitions from msvc.h, as they are not compiler related.

In mingw.h, determine the runtime version in use from the definitions of
_stati64 and _USE_32BIT_TIME_T, and define stat() accordingly.

This also fixes that stat() in MSVC builds still resolves to mingw_lstat()
instead of mingw_stat().

Signed-off-by: Karsten Blees <blees@dcon.de>
Acked-by: Sebastian Schuberth <sschuberth@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>