Added --with-sysinclude to allow explicitly setting where kernel includes

reside, so that you can easily switch between 2.0 and 2.2 ones.

Check existence of <linux/rtnetlink.h> for linux-22 configs to make sure
we're using the correct set of includes.

TODO

@@ -30,6 +30,8 @@ Core
 
 - socket: Use IP_RECVERR for BGP TCP sockets?
 
+- OSPF: refuse running on non-multicast devices
+
 - lib: use better checksum function
 
 Cleanup

configure.in

@@ -9,6 +9,7 @@ AC_CONFIG_AUX_DIR(tools)
 AC_ARG_ENABLE(debug,[  --enable-debug          enable internal debugging routines (default: enabled)],,enable_debug=yes)
 AC_ARG_WITH(sysconfig,[  --with-sysconfig=FILE   use specified BIRD system configuration file])
 AC_ARG_WITH(protocols,[  --with-protocols=LIST   include specified routing protocols (default: rip,static)],,[with_protocols="rip,static"])
+AC_ARG_WITH(sysinclude,[  --with-sysinclude=PATH  search for system includes on specified place])
 
 if test "$srcdir" = . ; then
 	# Building in current directory => create obj directory holding all objects
@@ -91,6 +92,9 @@ AC_MSG_CHECKING([what CFLAGS should we use])
 if test "$ac_test_CFLAGS" != set ; then
 	CFLAGS="$CFLAGS -Wall -W -Wstrict-prototypes -Wno-unused -Wno-parentheses"
 fi
+if test "$with_sysinclude" != no -a -n "$with_sysinclude"; then
+	CPPFLAGS="$CPPFLAGS -I$with_sysinclude"
+fi
 AC_MSG_RESULT($CFLAGS)
 
 AC_PROG_CPP
@@ -103,6 +107,11 @@ if test -z "$FLEX" -o -z "$BISON" -o -z "$M4" ; then
 	AC_MSG_ERROR([Some tools required for building BIRD are missing.])
 	fi
 
+case $sysdesc in
+	*/linux-22*)	AC_CHECK_HEADER(linux/rtnetlink.h,,[AC_MSG_ERROR([Appropriate version of Linux kernel headers not found.])])
+			;;
+esac
+
 AC_CHECK_HEADER(syslog.h, [AC_DEFINE(HAVE_SYSLOG)])
 
 BIRD_CHECK_INTEGERS