logfiles and absolute paths on windows

[petrelharp]

Over in https://github.com/popsim-consortium/stdpopsim/pull/1342 we've run into an issue that if you pass createLogFile an absolute path in Windows it erroneously interprets it as a relative path. Ben says that

I see the bug in log_file.cpp at line 61; it decides whether the path given by the user is an absolute path or not by checking whether the first character is '/' or not. You might put alternative logic in a #ifndef _WIN32 block there. See Eidos_ResolvedPath() at eidos_globals.cpp:1182 for a somewhat similar situation.