Build them if they don't exist, but don't care about them being
newer or older than the target.
I believe this is what was causing build loops where IPCCompiler was
being run a second time, rebuilding its .h file, then a library
would depend on that .h file and get re-archived, then an
application would need relinking, and something in that whole
process would trigger IPCCompiler running again touching its .h
file.