[lmi] xanadu/remove-ISOC99_SOURCE-def

[Greg Chicares]

I haven't applied this:

-// This selects a correct snprintf() for MinGW-w64.
-
-#if defined __GNUC__
-#   if !defined _ISOC99_SOURCE
-#       define _ISOC99_SOURCE
-#   endif // !defined _ISOC99_SOURCE
-#endif // defined __GNUC__

because, for the historical libmingwex at least,
some macro such as _ISOC99_SOURCE was required to
override a defective msvcrt snprintf(). IIRC, some
other macros served the same purpose; this one
seemed the least objectionable.

Do you know for sure that the MinGW-w64 project
has made that replacement unconditional?