bug#20681: Build failure [MSYS2/MINGW64, OSX]

[Eli Zaretskii <eliz@gnu.org>]

> Date: Thu, 28 May 2015 14:55:50 +0200
> From: Angelo Graziosi <angelo.graziosi@alice.it>
> 
> The build I tried with current master failed. This occurs in the 
> nextstep build on OSX and also in the MSYS2/MINGW64 on Windows. The same 
> master builds fine in the GTK build on GNU/Linux (but here there are 
> other issues; see bug #20677).
> 
> The failure looks the same.
> 
> On OSX:
> 
> [...]
>    GEN      dirent.h
>    GEN      fcntl.h
> /Applications/Xcode.app/Contents/Developer/usr/bin/make  all-am
>    CC       acl-errno-valid.o
>    CC       acl-internal.o
>    CC       get-permissions.o
>    CC       set-permissions.o
> set-permissions.c:569:20: warning: incompatible pointer to integer 
> conversion passing 'acl_t' (aka 'struct _acl *') to parameter of type 
> 'int' [-Wint-conversion]
>            acl = acl_init (acl);
>                            ^~~
> /usr/include/sys/acl.h:149:27: note: passing argument to parameter 
> 'count' here
> extern acl_t    acl_init(int count);
>                               ^
> set-permissions.c:585:26: error: use of undeclared identifier 'acl'
>          ret = acl_set_fd (desc, acl);
>                                  ^
> set-permissions.c:587:47: error: use of undeclared identifier 'acl'
>          ret = acl_set_file (name, ACL_TYPE_EXTENDED, acl);
>                                                       ^
> set-permissions.c:590:27: error: use of undeclared identifier 'saved_errno'
>            if (! acl_errno_valid (saved_errno) && ! 
> acl_extended_nontrivial (acl))
>                                   ^
> set-permissions.c:590:70: error: use of undeclared identifier 'acl'
>            if (! acl_errno_valid (saved_errno) && ! 
> acl_extended_nontrivial (acl))
>  
>       ^
> 1 warning and 4 errors generated.
> make[2]: *** [set-permissions.o] Error 1

Should be fixed now.  (set-permissions.c cannot be compiled on MinGW,
and is unnecessary there, so I removed it from the build.)

P.S. Thanks to Paul who made this fix easy by merging the gnulib
changes in nt/gnulib.mk.