[PATCH v2] mingw: avoid fallback for {local,gm}time_r()

From: =?UTF-8?q?Carlo=20Marcelo=20Arenas=20Bel=C3=B3n?= <carenas@gmail.com>

mingw-w64's pthread_unistd.h had a bug that mistakenly (because there is no support for the *lockfile() functions required[1]) defined _POSIX_THREAD_SAFE_FUNCTIONS and that was being worked around since 3ecd153a3b (compat/mingw: support MSys2-based MinGW build, 2016-01-14).

The bug was fixed in winphtreads, but as a side effect, leaves the reentrant functions from time.h no longer visible and therefore breaks the build.

Since the intention all along was to avoid using the fallback functions, formalize the use of POSIX by setting the corresponding feature flag and compile out the implementation for the fallback functions.

[1] https://unix.org/whitepapers/reentrant.html

Signed-off-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
Acked-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
---
    mingw: fix 64 bit build because of missing *time_r() function
    definitions
    
    This is a cherry-pick from git-for-windows/git#3398, that is needed to
    be able to build recent master with the Git for Windows SDK or recent
    MinGW64.
    
    It was discussed recently on list[1], and might need further
    adjustements if the 32-bit Git for Windows SDK also updates their
    winpthread headers, requiring a similar fix, but since without it a
    plain build from master wouldn't work in Windows I think it is worth
    reviewing on its own merits, and had withstood the test of time in the
    Git for Windows fork since it was originally merged there late August.
    
    Changes since v1:
    
     * Fixed grammar in the commit message (suggested by Junio)
     * Added ACK (proposed by dscho)
    
    [1] https://lore.kernel.org/git/20211005063936.588874-1-mh@glandium.org/

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-git-1142%2Fcarenas%2Fwinbuild-v2
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-git-1142/carenas/winbuild-v2
Pull-Request: https://github.com/git/git/pull/1142

Range-diff vs v1:

 1:  363b644f801 ! 1:  c6b3fc5a16a mingw: avoid fallback for {local,gm}time_r()
     @@ Commit message
          _POSIX_THREAD_SAFE_FUNCTIONS and that was being worked around since
          3ecd153a3b (compat/mingw: support MSys2-based MinGW build, 2016-01-14).
      
     -    the bug was fixed in winphtreads, but as a sideeffect, leaves the
     -    reentrant functions from time.h not longer visible and therefore breaks
     +    The bug was fixed in winphtreads, but as a side effect, leaves the
     +    reentrant functions from time.h no longer visible and therefore breaks
          the build.
      
     -    since the intention all along was to avoid using the fallback functions,
     +    Since the intention all along was to avoid using the fallback functions,
          formalize the use of POSIX by setting the corresponding feature flag and
     -    to make the intention clearer compile out the fallback functions.
     +    compile out the implementation for the fallback functions.
      
          [1] https://unix.org/whitepapers/reentrant.html
      
          Signed-off-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
     +    Acked-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
      
       ## compat/mingw.c ##
      @@ compat/mingw.c: int pipe(int filedes[2])

 compat/mingw.c    | 2 ++
 git-compat-util.h | 4 +++-
 2 files changed, 5 insertions(+), 1 deletion(-)