flockfile undefined on Mingw64 platform

[Daniel P. Berrange]

I'm trying to build libvirt with the latest GNULIB git master, on Mingw64
platform (Fedora 25 based).

I get compile warnings:

getopt.c: In function 'process_long_option':
getopt.c:281:5: warning: implicit declaration of function 'flockfile' 
[-Wimplicit-function-declaration]
     flockfile (stderr);
     ^~~~~~~~~
getopt.c:295:5: warning: implicit declaration of function 'funlockfile' 
[-Wimplicit-function-declaration]
     funlockfile (stderr);
     ^~~~~~~~~~~
gettimeofday.c:42:1: warning: 'initialize' defined but not used 
[-Wunused-function]
 initialize (void)
 ^~~~~~~~~~


and then link failures


../gnulib/lib/.libs/libgnu.a(getopt.o): In function `process_long_option':
/home/berrange/src/virt/libvirt/gnulib/lib/getopt.c:281: undefined reference to 
`flockfile'
/home/berrange/src/virt/libvirt/gnulib/lib/getopt.c:295: undefined reference to 
`funlockfile'
collect2: error: ld returned 1 exit status
Makefile:6361: recipe for target 'libvirt.la' failed


Looks like this is probably caused by this:

commit dcfe5a0e9cae998ae4e6480f357ff4af5e60fdf8
Author: Zack Weinberg <address@hidden>
Date:   Thu Apr 6 11:14:14 2017 -0700

    getopt: clean up error reporting
    
    getopt can print a whole bunch of error messages, and when used
    standalone (from gnulib) it uses fprintf to do that.  But fprintf is a
    cancellation point and getopt isn't, and also applying fprintf to a
    stream in wide-character mode is not allowed.  So every single error
    reporting case has an #ifdef _LIBC block in which it calls internal
    libc functions instead.  The counterpart patch series in glibc makes
    it possible to simplify all of that down to a set of #defines at the
    top of the file; core code is written as if it is safe to just call
    fprintf, flockfile, and funlockfile.  (One caveat: it's *not* safe to
    call any *other* stdio functions.)
    
    * lib/getopt.c: When _LIBC is defined, define fprintf to
    __fxprintf_nocancel, flockfile to _IO_flockfile, and funlockfile
    to _IO_funlockfile.  When neither _LIBC nor
    _POSIX_THREAD_SAFE_FUNCTIONS is defined, define flockfile and
    funlockfile as no-ops.
    (_getopt_internal_r): Remove all internal #ifdef _LIBC blocks; the
    standalone error-printing code can now be used for libc as well.
    Add an flockfile/funlockfile pair around one case where the error
    message is printed in several chunks.  Don't use fputc.

That code defines flockfile/funlockfile to no-ops, when
_POSIX_THREAD_SAFE_FUNCTIONS is *not* defined.

Unfortunately Mingw defines it in time.h and pthread_unistd.h, but does
*not* provide flockfile afaict.

Regards,
Daniel
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