All platform specific code in Realsoft 3D can be found in so called kernel library. The actual application code is built on top of this kernel layer and is therefore platform independent. Newer add any platform specific code, such as #ifdef WINDOWS, into your code.

Using #ifdef WINDOWS directive is not a smart thing to do anyway. WINDOWS means a huge set of different things and only Microsoft has control over them. Instead of using #ifdef WINDOWS directive you should use the directive that describes the feature in question only and nothing but the feature in question. For example, use #ifdef LITTLEENDIAN when you need to write byte ordering sensitive code instead of assuming that WINDOWS implies little-endian.

Realsoft 3D SDK uses platform independent makefiles. Don't put any platform specific definitions into your makefiles. If you have to, then define them in a separate platform specific makefile, and include the makefile into the main makefile using macros.