quieten unary argument warning during configure (#723)

shell variable can be empty

tools/ax_lib_readline.m4

@@ -57,22 +57,22 @@ if test $bash_cv_termcap_lib = gnutermcap && test -z "$prefer_curses"; then
 LDFLAGS="$LDFLAGS -L./lib/termcap"
 TERMCAP_LIB="./lib/termcap/libtermcap.a"
 TERMCAP_DEP="./lib/termcap/libtermcap.a"
-elif test $bash_cv_termcap_lib = libtermcap && test -z "$prefer_curses"; then
+elif test X$bash_cv_termcap_lib = Xlibtermcap && test -z "$prefer_curses"; then
 TERMCAP_LIB=-ltermcap
 TERMCAP_DEP=
-elif test $bash_cv_termcap_lib = libtinfow; then
+elif test X$bash_cv_termcap_lib = Xlibtinfow; then
 TERMCAP_LIB=-ltinfow
 TERMCAP_DEP=
-elif test $bash_cv_termcap_lib = libtinfo; then
+elif test X$bash_cv_termcap_lib = Xlibtinfo; then
 TERMCAP_LIB=-ltinfo
 TERMCAP_DEP=
-elif test $bash_cv_termcap_lib = libncursesw; then
+elif test X$bash_cv_termcap_lib = Xlibncursesw; then
 TERMCAP_LIB=-lncursesw
 TERMCAP_DEP=
-elif test $bash_cv_termcap_lib = libncurses; then
+elif test X$bash_cv_termcap_lib = Xlibncurses; then
 TERMCAP_LIB=-lncurses
 TERMCAP_DEP=
-elif test $bash_cv_termcap_lib = libc; then
+elif test X$bash_cv_termcap_lib = Xlibc; then
 TERMCAP_LIB=
 TERMCAP_DEP=
 else