build: fix host menu config targets using ncurses

On a recent Gentoo Linux installation, invoking `make menuconfig`, `make
kernel_menuconfig` or `make kernel_nconfig` in the build system fails,
whereas for example `make menuconfig` in the kernel tree alone works as
expected.

This is happening because STAGING_PREFIX is not defined when kernel's
{menu,n}config target calls pkg-config from the toolchain/host and thus
pkg-config returns an empty value, and the fallback values in the kernel
config script are applied but those are off and the linking fails.

Solution is to use system's pkg-config for all ncurses based menu config
targets in order to provide proper compiler/linker flags.

Ref: FS#2423
Cc: Thomas Albers <thomas.gameiro@gmail.com>
Tested-by: Stijn Tintel <stijn@linux-ipv6.be>
Tested-by: Eneas U de Queiroz <cotequeiroz@gmail.com>
Signed-off-by: Petr Štetiar <ynezz@true.cz>
(cherry picked from commit 965f341aa9)

Makefile

@@ -18,6 +18,7 @@ $(if $(findstring $(space),$(TOPDIR)),$(error ERROR: The path to the OpenWrt dir
 
 world:
 
+DISTRO_PKG_CONFIG:=$(shell which -a pkg-config | grep -E '\/usr' | head -n 1)
 export PATH:=$(TOPDIR)/staging_dir/host/bin:$(PATH)
 
 ifneq ($(OPENWRT_BUILD),1)

include/toplevel.mk

@@ -99,13 +99,18 @@ prepare-tmpinfo: FORCE
 		$(_SINGLE)$(NO_TRACE_MAKE) menuconfig $(PREP_MK); \
 	fi
 
+ifneq ($(DISTRO_PKG_CONFIG),)
+scripts/config/mconf: export PATH:=$(dir $(DISTRO_PKG_CONFIG)):$(PATH)
+endif
 scripts/config/mconf:
 	@$(_SINGLE)$(SUBMAKE) -s -C scripts/config all CC="$(HOSTCC_WRAPPER)"
 
 $(eval $(call rdep,scripts/config,scripts/config/mconf))
 
 scripts/config/qconf:
-	@$(_SINGLE)$(SUBMAKE) -s -C scripts/config qconf CC="$(HOSTCC_WRAPPER)"
+	@$(_SINGLE)$(SUBMAKE) -s -C scripts/config qconf \
+		CC="$(HOSTCC_WRAPPER)" \
+		DISTRO-PKG-CONFIG="$(DISTRO_PKG_CONFIG)"
 
 scripts/config/conf:
 	@$(_SINGLE)$(SUBMAKE) -s -C scripts/config conf CC="$(HOSTCC_WRAPPER)"
@@ -157,6 +162,10 @@ endif
 kernel_oldconfig: prepare_kernel_conf
 	$(_SINGLE)$(NO_TRACE_MAKE) -C target/linux oldconfig
 
+ifneq ($(DISTRO_PKG_CONFIG),)
+kernel_menuconfig: export PATH:=$(dir $(DISTRO_PKG_CONFIG)):$(PATH)
+kernel_nconfig: export PATH:=$(dir $(DISTRO_PKG_CONFIG)):$(PATH)
+endif
 kernel_menuconfig: prepare_kernel_conf
 	$(_SINGLE)$(NO_TRACE_MAKE) -C target/linux menuconfig
 

scripts/config/Makefile

@@ -20,8 +20,6 @@ check_lxdialog = $(shell $(SHELL) $(CURDIR)/lxdialog/check-lxdialog.sh -$(1))
 export CFLAGS += -DKBUILD_NO_NLS -I. $(call check_lxdialog,ccflags)
 export CXXFLAGS += -DKBUILD_NO_NLS
 
-DISTRO-PKG-CONFIG := $(shell which -a pkg-config | grep -E '\/usr' | head -n 1)
-
 conf-objs	:= conf.o zconf.tab.o
 mconf-objs	:= mconf.o zconf.tab.o
 qconf-cxxobjs	:= qconf.o