Use default cpp fmt build script.

vendor/Fmt/test/posix-mock.h

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+// Formatting library for C++ - mocks of POSIX functions
+//
+// Copyright (c) 2012 - present, Victor Zverovich
+// All rights reserved.
+//
+// For the license information refer to format.h.
+
+#ifndef FMT_POSIX_TEST_H
+#define FMT_POSIX_TEST_H
+
+#include <errno.h>
+#include <locale.h>
+#include <stdio.h>
+#ifdef __APPLE__
+#  include <xlocale.h>
+#endif
+
+#ifdef _WIN32
+#  include <windows.h>
+#else
+#  include <sys/param.h>  // for FreeBSD version
+#  include <sys/types.h>  // for ssize_t
+#endif
+
+#ifndef _MSC_VER
+struct stat;
+#endif
+
+namespace test {
+
+#ifndef _MSC_VER
+// Size type for read and write.
+typedef size_t size_t;
+typedef ssize_t ssize_t;
+int open(const char* path, int oflag, int mode);
+int fstat(int fd, struct stat* buf);
+#else
+typedef unsigned size_t;
+typedef int ssize_t;
+errno_t sopen_s(int* pfh, const char* filename, int oflag, int shflag,
+                int pmode);
+#endif
+
+#ifndef _WIN32
+long sysconf(int name);
+#else
+DWORD GetFileSize(HANDLE hFile, LPDWORD lpFileSizeHigh);
+#endif
+
+int close(int fildes);
+
+int dup(int fildes);
+int dup2(int fildes, int fildes2);
+
+FILE* fdopen(int fildes, const char* mode);
+
+ssize_t read(int fildes, void* buf, size_t nbyte);
+ssize_t write(int fildes, const void* buf, size_t nbyte);
+
+#ifndef _WIN32
+int pipe(int fildes[2]);
+#else
+int pipe(int* pfds, unsigned psize, int textmode);
+#endif
+
+FILE* fopen(const char* filename, const char* mode);
+int fclose(FILE* stream);
+int(fileno)(FILE* stream);
+
+#if defined(FMT_LOCALE) && !defined(_WIN32)
+locale_t newlocale(int category_mask, const char* locale, locale_t base);
+#endif
+}  // namespace test
+
+#define FMT_SYSTEM(call) test::call
+
+#endif  // FMT_POSIX_TEST_H