m4: readline: Drop extra_LIBS machinery

The first implementation of this logic was introduced with
commit 2ec759fc all the way back in 2007; looking at the
build logs from our CI environment, however, it's apparent
that none of the platforms we currently target are actually
using it, so we can assume whatever issue it was working
around has been fixed at some point in the last 12 years.
Signed-off-by: NAndrea Bolognani <abologna@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>

m4/virt-readline.m4

@@ -22,20 +22,6 @@ AC_DEFUN([LIBVIRT_ARG_READLINE],[
 ])
 
 AC_DEFUN([LIBVIRT_CHECK_READLINE],[
-  extra_LIBS=
-  lv_saved_libs=$LIBS
-  if test "x$with_readline" != xno; then
-    # Linking with -lreadline may require some termcap-related code, e.g.,
-    # from one of the following libraries.  Add it to LIBS before using
-    # canned library checks; then verify later if it was needed.
-    LIBS=
-    AC_SEARCH_LIBS([tgetent], [ncurses curses termcap termlib])
-    case $LIBS in
-      no*) ;;  # handle "no" and "none required"
-      *) extra_LIBS=$LIBS ;; # anything else is a -lLIBRARY
-    esac
-    LIBS="$lv_saved_libs $extra_LIBS"
-  fi
 
   # This function is present in all reasonable (5.0+) readline versions;
   # however, the macOS base system contains a library called libedit which
@@ -59,14 +45,6 @@ AC_DEFUN([LIBVIRT_CHECK_READLINE],[
   # The normal library check...
   LIBVIRT_CHECK_LIB([READLINE], [readline], [readline], [readline/readline.h])
 
-  # Touch things up to avoid $extra_LIBS, if possible.  Test a second
-  # function, to ensure we aren't being confused by caching.
-  LIBS=$lv_saved_libs
-  AC_CHECK_LIB([readline], [rl_initialize],
-               [],
-               [READLINE_LIBS="$READLINE_LIBS $extra_LIBS"])
-  LIBS=$lv_saved_libs
-
   # We need this to avoid compilation issues with modern compilers.
   # See 9ea3424a178 for a more detailed explanation
   if test "$with_readline" = "yes" ; then