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.

[This patch is already applied upstream (is part of linux v3.18):
https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=be8af2d54a66911693eddc556e4f7a866670082b



I'm adding this instead of doing a full upstream kconfig sync because
there was a conflict in one of the Buildroot kconfig patches (against
linux 3.18-rc1), which I was unable to resolve. Just drop this patch next time
Buildroot kconfig is synced against upstream.
]

Signed-off-by: Bjørn Forsman <bjorn.forsman@gmail.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>