Fix FMT_OS definition

[phprus]

If cmake option FMT_OS is OFF then define FMT_OS=0.
If cmake option FMT_OS is ON then FMT_OS is not defined by cmake and defined in fmt.cc:

fmt/src/fmt.cc

Lines 86 to 88 in 6c3b2d4

	#ifndef FMT_OS
	# define FMT_OS 1
	#endif

issue: #3777

[vitaut]

Good catch, thanks!