kconfig: if ncurses-devel is missing then say so

With this patch when ncurses-devel (or whatever it is named)
is missing trying to run menuconfig will result in this:

$ make menuconfig
  HOSTCC  scripts/kconfig/conf.o
  HOSTCC  scripts/kconfig/kxgettext.o
 *** Unable to find the ncurses libraries or the
 *** required header files.
 *** 'make menuconfig' requires the ncurses libraries.
 ***
 *** Install ncurses (ncurses-devel) and try again.
 ***
make[1]: *** [scripts/kconfig/dochecklxdialog] Error 1
make: *** [menuconfig] Error 2

Much better than before where we just listed some build errors.
The other *config targets will work indepenednt on ncurses
being present or not.

Includes improvements suggested by: Frans Pop <elendil@planet.nl>

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Frans Pop <elendil@planet.nl>