mingw32: fix windows-termcap/curses check

mingw32: fix windows-termcap/curses check

When GDB is configured with "--without-tui --with-curses" or "--with-tui",
$prefer_curses is set to yes.  But, that still doesn't mean that curses
will be used.  configure will still search for the curses library, and
continue building without it.  That's done here:

 curses_found=no
 if test x"$prefer_curses" = xyes; then
 ...
   AC_SEARCH_LIBS(waddstr, [ncurses cursesX curses])

   if test "$ac_cv_search_waddstr" != no; then
     curses_found=yes
   fi
 fi

So if waddstr is not found, meaning curses is not really
available, even though it'd be preferred, $prefer_curses is
'yes', but $curses_found is 'no'.

So the right fix to tell whether we're linking with curses is
$curses_found=yes.

gdb/ChangeLog:
2015-01-22  Pedro Alves  <palves@redhat.com>

	* configure.ac [*mingw32*]: Check $curses_found instead of
	$prefer_curses.
	* configure: Regenerate.
	* windows-termcap.c: Remove HAVE_CURSES_H, HAVE_NCURSES_H and
	HAVE_NCURSES_NCURSES_H checks.