os.h should accept a predefined FMT_USE_FCNTL

[timkalu]

os.h should accept a predefined FMT_USE_FCNTL override so using FMT with e.g. the NXP toolchain for ARM (under FreeRTOS) does not have a fnctl call.

How about this:
os.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))
#  i  nclude <fcntl.h>  // for O_RDONLY
#    define FMT_USE_FCNTL 1
#  else
#    define FMT_USE_FCNTL 0
#  endif
#endif

The detection of FMT_HAS_INCLUDE does not work correctly for this toolchain.
(see also #2277)

[vitaut]

Sounds reasonable, a PR would be welcome.