Add additional processing in an automake build for one object

I'm trying to make a small change to an automake build. The system to modify uses configure.ac and Makefile.am inputs. For a single object file within one subdirectory I have to invoke a script before compiling, to patch config info into the build. I don't see the right location to allow such pre-processing ahead of compiling this specific C file. What I've tried is to insert an additional target into all: all-am But this seems not to be the way to go and in addition I wasn't able to figure how to overload this generated line.