Factor crash with gcc 4.3.0

If you are running the following version of gcc on Linux/x86, Factor might crash at the beginning of stage2 bootstrap:

gcc (Debian 4.3-20080202-1) 4.3.0 20080202 (experimental)

This seems to be a gcc bug. There is a workaround, however; it was discovered by jamesvnc of #concatenative. Just pass the following argument to make:

make SITE_CFLAGS=-fno-forward-propagate