mingw: use strftime() directly in UCRT builds

The `mingw_strftime()` wrapper exists to work around msvcrt.dll's
incomplete `strftime()` implementation by dynamically loading the
version from ucrtbase.dll at runtime via `LoadLibrary()` +
`GetProcAddress()`. When the binary is already linked against UCRT
(i.e. when building in the UCRT64 environment), the linked-in
`strftime()` is the ucrtbase.dll version, making the dynamic loading
needless churn: It's calling the very same code.

Simply guard both the declaration and implementation so that the
unnecessary work-around is skipped in UCRT builds.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

compat/mingw-posix.h

@@ -411,9 +411,11 @@ extern int (*lstat)(const char *file_name, struct stat *buf);
 
 int mingw_utime(const char *file_name, const struct utimbuf *times);
 #define utime mingw_utime
+#ifndef _UCRT
 size_t mingw_strftime(char *s, size_t max,
 		   const char *format, const struct tm *tm);
 #define strftime mingw_strftime
+#endif
 
 pid_t mingw_spawnvpe(const char *cmd, const char **argv, char **env,
 		     const char *dir,

compat/mingw.c

@@ -1505,6 +1505,7 @@ int mingw_utime (const char *file_name, const struct utimbuf *times)
 	return rc;
 }
 
+#ifndef _UCRT
 #undef strftime
 size_t mingw_strftime(char *s, size_t max,
 		      const char *format, const struct tm *tm)
@@ -1524,6 +1525,7 @@ size_t mingw_strftime(char *s, size_t max,
 		die("invalid strftime format: '%s'", format);
 	return ret;
 }
+#endif
 
 unsigned int sleep (unsigned int seconds)
 {