Well, if you have the sources of fpc (as you should have) it's not that hard.
make clean OPT="-g -gl -O-"
make install
It will install the newly generated fpc in the defualt location (c:\pp in Windows, no idea what it is on *.nix, where you also must have root privileges).
And the new fpc must have a fpc.cfg as well.
In the folder where the binary is (fpc.exe in windows, fpc in *.nix) do:
fpcmkcfg -o fpc.cfg
Bart