[RFA] AIX thread support

[David Edelsohn]

	If you dislike threads-aix.h, then the alternative is to determine
threads-posix.h versus threads-no.h at configure time:

  dnl Check for thread package actually supported in libstdc++ 
  THREADH=
  case "$target_thread_file" in
    no | none | single)
      THREADH=threads-no.h
      ;;
    aix)
      case "$CXXFLAGS" in
        *pthread*)
          THREADH=threads-posix.h
          ;;
        *)
          THREADH=threads-no.h
          ;;
      esac
      ;;
    posix | pthreads)
      THREADH=threads-posix.h
      ;;
    decosf1 | irix | mach | os2 | solaris | win32 | dce | vxworks)
      AC_MSG_ERROR(thread package $THREADS not yet supported)
      ;;
    *)
      AC_MSG_ERROR($THREADS is an unknown thread package)
      ;;
  esac
  AC_MSG_RESULT($THREADH)

Is the above approach more acceptable?

David