Fix quote around macro argument

[rrebello]

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.

[dequis]

Somehow didn't see this one before. Tested and looks good. Thanks!