build: configure "--with-curses=yes" is now a no-op

It makes no sense to error out if the builder runs "./configure
--with-curses" and no option argument. "--with-curses" without an option
argument now keeps using the default name list for detecting ncurses
library.

Signed-off-by: Kang-Che Sung <explorer09@gmail.com>
diff --git a/configure.ac b/configure.ac
index 9b81c3f..19fcccc 100644
--- a/configure.ac
+++ b/configure.ac
@@ -412,10 +412,10 @@
             [],
             [with_curses=check])
 case $with_curses in
-check)
+check|yes)
    : # No-op. Use default list.
    ;;
-yes|no)
+no)
    AC_MSG_ERROR([bad value '$with_curses' for --with-curses option])
    ;;
 *)