kconfig/lxdialog: rationalise the include paths where to find {.n}curses{,w}.h

kconfig/lxdialog: rationalise the include paths where to find {.n}curses{,w}.h

The current code does this:

    if [ -f /usr/include/ncursesw/curses.h ]; then
        echo '-I/usr/include/ncursesw -DCURSES_LOC="<ncursesw/curses.h>"'
    elif [ -f /usr/include/ncurses/ncurses.h ]; then
        echo '-I/usr/include/ncurses -DCURSES_LOC="<ncurses.h>"'
    elif [ -f /usr/include/ncurses/curses.h ]; then
        echo '-I/usr/include/ncurses -DCURSES_LOC="<ncurses/curses.h>"'
    [...]

This is merely inconsistent:
  - adding the full path to the directory in the -I directive,
  - especially since that path is already a sub-path of the system
    include path,
  - and then repeating the sub-path in the #include directive.

Rationalise each include directive:
  - only use the filename in the #include directive,
  - keep the -I directives: they are always searched for before the
    system include path; this ensures the correct header is used.

Using the -I directives and the filename-only in #include is more in
line with how pkg-config behaves, eg.:
    $ pkg-config --cflags ncursesw
    -I/usr/include/ncursesw

This paves the way for using pkg-config for CFLAGS, too, now we use it
to find the libraries.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>