Fix quote around macro argument

In m4/curses.m4, line 134, the 5th argument passed to AC_NCURSES was
surrounded by '"' instead of '[' and ']'. Because of that, the expansion
of AC_NCURSES in that case would produce the following line inside the
configure script (note the repeated double quotes):

  screen_manager=""ncurses on $withval/include""

That would cause the following error when configure was executed with
the "--with-ncurses=dir" argument:

  ./configure: line 13468: on: command not found

Although in the case above the error doesn't actually influence the
build process ('screen_manager' isn't used anywhere in the script),
trying to execute 'on' might be harmful if it corresponded to an
existing command in the user's environment.

m4/curses.m4

@@ -131,7 +131,7 @@ AC_DEFUN([AC_CHECK_CURSES],[
 	  if test x$withval = xno ; then
 		search_ncurses=false
 	  elif test x$withval != xyes ; then
-		AC_NCURSES($withval/include, ncurses.h, -L$withval/lib -lncurses, -I$withval/include, "ncurses on $withval/include")
+		AC_NCURSES($withval/include, ncurses.h, -L$withval/lib -lncurses, -I$withval/include, [ncurses on $withval/include])
 	  fi
 	)