pppd: fix build with 3.5+ kernel headers

Fixes http://autobuild.buildroot.net/results/8a9369fece696e012249b865708ffee533c5318a

Pppd bundles some, but not all needed kernel headers. In Linux 3.5 (and
before that 2.6.35) if_pppol2tp.h was changed, so the embedded copy was no
longer compatible.

if_pppol2tp.h has been part of the exported kernel headers since 2.6.23, so
simply drop the bundled copy and use the kernel headers instead, similar
to how it is done in Gentoo:

https://bugs.gentoo.org/show_bug.cgi?id=427684

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/pppd/pppd-2.4.5-ol2tpv3-2.6.35.patch

@@ -1,24 +0,0 @@
---- ppp-2.4.5/include/linux/if_pppol2tp.h       2009-11-16 22:26:07.000000000 +0000
-+++ ppp-2.4.5/include/linux/if_pppol2tp.h       2010-07-16 22:35:22.000000000 +0100
-@@ -32,6 +32,20 @@
-  	__u16 d_tunnel, d_session;	/* For sending outgoing packets */
- };
-
-+/* The L2TPv3 protocol changes tunnel and session ids from 16 to 32
-+ * bits. So we need a different sockaddr structure.
-+ */
-+struct pppol2tpv3_addr {
-+       pid_t   pid;                    /* pid that owns the fd.
-+                                        * 0 => current */
-+       int     fd;                     /* FD of UDP or IP socket to use */
-+
-+       struct sockaddr_in addr;        /* IP address and port to send to */
-+
-+       __u32 s_tunnel, s_session;      /* For matching incoming packets */
-+       __u32 d_tunnel, d_session;      /* For sending outgoing packets */
-+};
-+
- /* Socket options:
-  * DEBUG	- bitmask of debug message categories
-  * SENDSEQ	- 0 => don't send packets with sequence numbers
-

package/pppd/pppd.mk

@@ -23,6 +23,15 @@ ifeq ($(BR2_INET_IPV6),y)
 	PPPD_MAKE_OPT += HAVE_INET6=y
 endif
 
+# pppd bundles some but not all of the needed kernel headers. The embedded
+# if_pppol2tp.h is unfortunately not compatible with kernel headers > 2.6.34,
+# and has been part of the kernel headers since 2.6.23, so drop it
+define PPPD_DROP_INTERNAL_IF_PPOL2TP_H
+	$(RM) $(@D)/include/linux/if_pppol2tp.h
+endef
+
+PPPD_POST_EXTRACT_HOOKS += PPPD_DROP_INTERNAL_IF_PPOL2TP_H
+
 define PPPD_CONFIGURE_CMDS
 	$(SED) 's/FILTER=y/#FILTER=y/' $(PPPD_DIR)/pppd/Makefile.linux
 	$(SED) 's/ifneq ($$(wildcard \/usr\/include\/pcap-bpf.h),)/ifdef FILTER/' $(PPPD_DIR)/*/Makefile.linux