So... I'm finally looking at the source for BII, and it appears that building on OS X is totally undocumented since the code rearranging; the best comments I've got on how to do it are out of the changelog.
In there, it say sto go to the Unix folder, do autoconf, autoheader and then configure --enable-macosx-gui. However, this seems to be a bit lacking, as autoconf and configure both throw errors.
No matter what configure flags I use, I get:
./configure: line 7302: syntax error near unexpected token `1.3.15,'
./configure: line 7302: ` AM_PATH_GTK_2_0(1.3.15,'
This seems to indicate it is trying to compile against GTK 2, which is odd, as I enabled macosx gui and disabled X. I tried with SDL and without.