Ben,
I DID use gcc before I ever tried clang. I just verified that it also worked with clang.
Octave builds on OSX for me with
a) 32 bit using gcc 4.2x from Apple, gfortran 4.2.x from ATT, with -ff2c flag, without BLAS wrapper, without FLTK, without -D__LISTS__
b) 64 bit using gcc 4.2.x from Apple, gfortran 4.2.x from ATT, without -ff2c flag, with BLAS wrapper, without FLTK, without -D__LISTS__
c) 64 bit using clang from Apple, gfortran 4.2.x from ATT, without -ff2c flag, with BLAS wrapper, without FLTK, without -D__LISTS__
Campbell