Relying on the preprocessor and predefined compiler macros for achieving portability seems hard to manage. What's a better way to achieve portability for a C project? I want to put environment-specific code in headers that behave the same way. Is there a way to have the build environment choose which headers to include?

I was thinking that I'd put the environment-specific headers into directories for specific environments. The build environment would then just copy the headers from the platform's directory into the root directory, build the project, and then remove the copies.