I can't help you directly, but I can prepare you for what to expect from Informix in this regard: Very little, unless you want to pay for it. We ran into the same problem with Red Hat 7.3, which ships with gcc 2.96
Informix claimed compatibility with a variety of Red Hat releases, including 7.3, but then it turned out that the part of the SDK we were interested in (support for a little obscure language called C++) was actually incompatible with gcc 2.96. Rather than privately fund a patch solution, we turned to RogueWave SourcePro, which uses the same API as the ODBC layer, and have had some success there.
Multi-thread safety is critical to us- I am not sure what requirements are driving you, but at least this may be an option.
A caveat for any of the SDK's is: try to get all of your requirements fleshed out up front. We have RogueWave isolated inside our data tier in order to manage the cost of developer seats, and now we have some compatibility issues between it and the multi-thread STL we selected early on for the entire project team (STLport). We're still trying to work through that, so if you have similar concerns, ask questions early on, and get answers in writing.