Windows (under Cygwin)
This is usually code for "needs UNIX/POSIX APIs", or its uglier cousin, "uses C99". I wouldn't mind Mingw32, and would mind a little bit Cygwin, but the problem is that for a video game I can't even begin to consider those platforms for Windows. All the official platform SDKs, or the third party ones (think Steam) require the Microsoft compilers and libs.
Some of the optional extension libraries like networking may not work out of the box, but TinyScheme doesn't have these anyway.
Basically, Chibi was designed as a better TinyScheme. It's just as small, can also easily be included statically without a library, has a nicer (optional) FFI, various features like full Unicode, full numeric tower and hygienic macros (all optional), and is an order of magnitude faster.
http://bellecrazysnail.wordpress.com/2013/11/14/some-thought...
There's also some weird double personality issues going on with their compilers in MSVC2013, for example they support designated initializers in plain C but not in C++. So you get partial C99 inside pure C or C89 (MS flavored) under C++.
I forgot where I read the quote, but for a decade Microsoft saw the plain C compiler as tool exclusively meant for Windows development. While gcc and later clang adopted and embraced new C idioms, like the struct syntax I mention, they kept their plain C support frozen in time.
I also found weird behavior in MSVC when pushing the C preprocessor VERY hard (nested variadic macros. That would require an entire blog post for itself...)