C extensions, portability, and alternative compilers

Anyone who's written C knows that full ISO C standard-adhering code is an impractical rarity. Most real world C code out there relies on non-standard behaviors and language extensions to varying extents, and a lot of this isn't for extra features, but just to work around bugs and gaps in different compilers and libraries. A lot of codebases will try somewhat to support various environments, mostly through the use of preprocessor checks and guards, but these attempts are finicky at best and...