Note also, someone failed with the same error you had using "gcc4
from Darwin ports":
http://lists.gnu.org/archive/html/discuss-gnustep/2005-07/
msg00002.html
And again on Panther:
http://lists.gnu.org/archive/html/bug-gnustep/2005-06/msg00135.html
No solution found in either case, though there are periodic success
reports as well. I suspect there must be some condition that
hasn't been identified and put in the instructions that affects
finding the objc library during linking. Maybe this condition
depends on OS X and compiler versions, maybe not..