In configure.in (after $target def' of course) :

case "${target}" in
  *ppc*)
    libdetect_targetdep_header=ppc.h
    ;;
  *i386*) # may be incompete
    libdetect_targetdep_header=i386
    ;;
  # ...
esac

about the end of configure.in file (near other AC_SUBST)

AC_SUBST(libdetect_targetdep_header)