kconfig/lxdialog: get ncurses CFLAGS with pkg-config

This makes "make menuconfig" also work on systems where ncurses is not
installed in a standard location (such as on NixOS).

This patch changes ccflags() so that it tries pkg-config first, and only
if pkg-config fails does it go back to the fallback/manual checks. This
is the same algorithm that ldflags() already uses.

Signed-off-by: Bjørn Forsman <bjorn.forsman@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>