mingw: avoid fallback for {local,gm}time_r()

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
3ecd153 (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 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
to make the intention clearer compile out the fallback functions.

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

Signed-off-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>

compat/mingw.c

@@ -1298,6 +1298,7 @@ int pipe(int filedes[2])
 	return 0;
 }
 
+#ifndef __MINGW64__
 struct tm *gmtime_r(const time_t *timep, struct tm *result)
 {
 	if (gmtime_s(result, timep) == 0)
@@ -1311,6 +1312,7 @@ struct tm *localtime_r(const time_t *timep, struct tm *result)
 		return result;
 	return NULL;
 }
+#endif
 
 char *mingw_strbuf_realpath(struct strbuf *resolved, const char *path)
 {

git-compat-util.h

@@ -127,7 +127,9 @@
 /* Approximation of the length of the decimal representation of this type. */
 #define decimal_length(x)	((int)(sizeof(x) * 2.56 + 0.5) + 1)
 
-#if defined(__sun__)
+#ifdef __MINGW64__
+#define _POSIX_C_SOURCE 1
+#elif defined(__sun__)
  /*
   * On Solaris, when _XOPEN_EXTENDED is set, its header file
   * forces the programs to be XPG4v2, defeating any _XOPEN_SOURCE