Building in Cygwin fails due to undeclared strptime in time.h

[thesmurph]

Building in Cygwin fails due to undeclared strptime in time.h. This function is not available when __GNU_VISIBLE is not defined. The strptime() function is defined in the X/Open Portability Guide Issue 4 ("XPG4"). Ultimately to enable strptime you can either define _GNU_SOURCE or set _XOPEN_VERSION to 4/500/600/700. I was able to build fmt after adding "add_definitions(-D_XOPEN_SOURCE=700)" to CMakeLists.txt.

CMake version: 3.23.2
The CXX compiler identification is GNU 11.3.0
commit d2e89c8

Error Message
[ 68%] Building CXX object test/CMakeFiles/scan-test.dir/scan-test.cc.o
/home/Matt/fmt/test/scan-test.cc: In instantiation of ‘typename ScanContext::iterator fmt::v9::scanner::scan(tm&, ScanContext&) [with ScanContext = fmt::v9::scan_context; typename ScanContext::iterator = const char*]’:
/home/Matt/fmt/test/scan.h:111:26: required from ‘static void fmt::v9::detail::scan_arg::scan_custom_arg(void*, fmt::v9::scan_parse_context&, fmt::v9::scan_context&) [with T = tm]’
/home/Matt/fmt/test/scan.h:102:17: required from ‘fmt::v9::detail::scan_arg::scan_arg(T&) [with T = tm]’
/home/Matt/fmt/test/scan.h:231:20: required from ‘std::array<fmt::v9::detail::scan_arg, sizeof... (Args)> fmt::v9::make_scan_args(Args& ...) [with Args = {tm}]’
/home/Matt/fmt/test/scan.h:244:49: required from ‘const char* fmt::v9::scan(fmt::v9::string_view, fmt::v9::string_view, Args& ...) [with Args = {tm}; fmt::v9::basic_string_view::iterator = const char*; fmt::v9::string_view = fmt::v9::basic_string_view]’
/home/Matt/fmt/test/scan-test.cc:96:12: required from here
/home/Matt/fmt/test/scan-test.cc:86:27: error: ‘strptime’ was not declared in this scope; did you mean ‘strftime’?
86 | auto result = strptime(ctx.begin(), format.c_str(), &t);
| ~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
| strftime
make[2]: *** [test/CMakeFiles/scan-test.dir/build.make:76: test/CMakeFiles/scan-test.dir/scan-test.cc.o] Error 1
make[1]: *** [CMakeFiles/Makefile2:590: test/CMakeFiles/scan-test.dir/all] Error 2
make: *** [Makefile:166: all] Error 2

/usr/include/time.h
#if __GNU_VISIBLE
char *strptime_l (const char *__restrict, const char *__restrict,
struct tm *__restrict, locale_t);
#endif

cygdrive/c/cygwin64/usr/include/sys/features.h
#ifdef _GNU_SOURCE
#define __GNU_VISIBLE 1
#else
#define __GNU_VISIBLE 0
#endif

[vitaut]

strptime is only used in scan-test. A PR to conditionally disable that test would be welcome.