FMT_USE_FCNTL can be predefined (#2573)

'os.h' accepts a predefined FMT_USE_FCNTL override so using FMT with e.g. the NXP toolchain for ARM (e.g. for FreeRTOS) does not have a fcntl() call, but the detection routine does not detect this correctly.

include/fmt/os.h

@@ -21,17 +21,19 @@
 
 #include "format.h"
 
+#ifndef FMT_USE_FCNTL
 // UWP doesn't provide _pipe.
-#if FMT_HAS_INCLUDE("winapifamily.h")
-#  include <winapifamily.h>
-#endif
-#if (FMT_HAS_INCLUDE(<fcntl.h>) || defined(__APPLE__) || \
-     defined(__linux__)) &&                              \
-    (!defined(WINAPI_FAMILY) || (WINAPI_FAMILY == WINAPI_FAMILY_DESKTOP_APP))
+#  if FMT_HAS_INCLUDE("winapifamily.h")
+#    include <winapifamily.h>
+#  endif
+#  if (FMT_HAS_INCLUDE(<fcntl.h>) || defined(__APPLE__) || \
+       defined(__linux__)) &&                              \
+      (!defined(WINAPI_FAMILY) || (WINAPI_FAMILY == WINAPI_FAMILY_DESKTOP_APP))
 #  include <fcntl.h>  // for O_RDONLY
-#  define FMT_USE_FCNTL 1
-#else
-#  define FMT_USE_FCNTL 0
+#    define FMT_USE_FCNTL 1
+#  else
+#    define FMT_USE_FCNTL 0
+#  endif
 #endif
 
 #ifndef FMT_POSIX