iso8601: implemented fallback for gmtime_r() on WIN32
iso8601: implemented fallback for gmtime_r() on WIN32

It's not exactly a valid callback, because it's not thread-safe, but
that's the best we can do now.

cmende/libmpdclient,jagleeso/libmpdclient,jagleeso/libmpdclient,yusiwen/libmpdclient,sl0th/libmpdclient,psych0tik/libmpdclient2,jwise/libmpdclient,OpenInkpot-archive/iplinux-libmpdclient,sl0th/libmpdclient,jwise/libmpdclient,psych0tik/libmpdclient2,cmende/libmpdclient,yusiwen/libmpdclient,OpenInkpot-archive/iplinux-libmpdclient,yusiwen/libmpdclient,jagleeso/libmpdclient,jwise/libmpdclient,sl0th/libmpdclient,cmende/libmpdclient