Many binutils versions over the last year appear to silently build
assembler files with undefined constants, and able to successfully
create executables from such files.  The assembler appears to add
undefined symbols to the symbol table without any corresponding
relocation information.  Obviously this is bad news since the
resulting executable may not be what the programmer intended.
Work around the problem by forcing the linker to fail if there are
any undefined symbols in the final object(s).