Gnash is now completely compilable and optimizable with the LLVM frontend Clang. The AGG renderer headers have one C++ bug that causes an error. This needs to be fixed externally if you want to use that renderer, as it seems legitimate for a compiler to reject it (even if GCC doesn't).
Clang has already helped to find a few bugs in Gnash. Some warnings picked up things that GCC missed. And most interestingly, there were cases where Gnash's behaviour was relying on the order of evaluating function arguments.