Ah, I nm'd all files in src/interfaces/libpq and got no hits for abort. But I did get one in libpgport_shlib.a:
path_shlib.o: U abort 0000000000000320 T canonicalize_path 0000000000000197 T cleanup_path 00000000000009e3 t dir_strcmp ... -- Álvaro Herrera Valdivia, Chile "People get annoyed when you try to debug them." (Larry Wall)