busybox: Fix pam_start link errors when PAM support is enabled

Patch to BusyBox Makefile to fix linker errors when PAM support is enabled by
CONFIG_BUSYBOX_CONFIG_PAM=y. Typical errors include:
login.c:(.text.login_main+0x122): undefined reference to `pam_start'
Signed-off-by: Stephen Parry <sgparry@mainscreen.com>

Backport of r42195

git-svn-id: svn://svn.openwrt.org/openwrt/branches/barrier_breaker@42368 3c298f89-4303-0410-b956-a3cf2f4a3e73

package/utils/busybox/Makefile

@@ -17,7 +17,7 @@ PKG_SOURCE_URL:=http://www.busybox.net/downloads \
 		http://distfiles.gentoo.org/distfiles/
 PKG_MD5SUM:=337d1a15ab1cb1d4ed423168b1eb7d7e
 
-PKG_BUILD_DEPENDS:=BUSYBOX_USE_LIBRPC:librpc
+PKG_BUILD_DEPENDS:=BUSYBOX_USE_LIBRPC:librpc BUSYBOX_CONFIG_PAM:libpam
 PKG_BUILD_PARALLEL:=1
 PKG_CHECK_FORMAT_SECURITY:=0
 
@@ -42,7 +42,7 @@ define Package/busybox
   MAINTAINER:=Felix Fietkau <nbd@openwrt.org>
   TITLE:=Core utilities for embedded Linux
   URL:=http://busybox.net/
-  DEPENDS:=+BUSYBOX_USE_LIBRPC:librpc
+  DEPENDS:=+BUSYBOX_USE_LIBRPC:librpc +BUSYBOX_CONFIG_PAM:libpam
   MENU:=1
 endef
 
@@ -80,6 +80,12 @@ ifdef CONFIG_BUSYBOX_USE_LIBRPC
   LDLIBS += rpc
 endif
 
+ifdef CONFIG_BUSYBOX_CONFIG_PAM
+  TARGET_CFLAGS += -I$(STAGING_DIR)/usr/include
+  export LDFLAGS=$(TARGET_LDFLAGS)
+  LDLIBS += pam pam_misc pthread
+endif
+
 define Build/Compile
 	+$(MAKE) $(PKG_JOBS) -C $(PKG_BUILD_DIR) \
 		CC="$(TARGET_CC)" \