I tried using sum2.diff, but it caused make to look for a non-existent object file, so it doesn't appear to be necessary under Fedora. I therefore backed it out. Thereon I rebuilt using the system GCC and searched for something under the built directory with a name resembling "libgpc.a". It doesn't exist.
$ find . -name "*gpc*"
./gcc/gpc1
./gcc/xgpc
./gcc/gpc-run
./gcc/p/gpcpp.o
./gcc/p/gpc-lex.o
./gcc/p/gpc.o
./prev-gcc/gpc1
./prev-gcc/xgpc
./prev-gcc/gpc-run
./prev-gcc/p/gpcpp.o
./prev-gcc/p/gpc-lex.o
./prev-gcc/p/gpc.o
So it looks like I get to give clang a shot.