isisd: change ISIS_METHOD to use C preprocessor

this fixes warnings from vtysh extract.pl by making sure the isis method
files always 'work'. (previously, extract.pl would grab unselected isis
method sources and then complain about missing headers)
diff --git a/isisd/Makefile.am b/isisd/Makefile.am
index 9c30339..26b8ee7 100644
--- a/isisd/Makefile.am
+++ b/isisd/Makefile.am
@@ -13,8 +13,6 @@
 sbin_PROGRAMS = isisd 
 SUBDIRS = topology
 
-isis_method = @ISIS_METHOD@
-
 libisis_a_SOURCES = \
 	isis_adjacency.c isis_lsp.c dict.c isis_circuit.c isis_pdu.c \
 	isis_tlv.c isisd.c isis_misc.c isis_zebra.c isis_dr.c \
@@ -30,13 +28,10 @@
 	include-netbsd/clnp.h include-netbsd/esis.h include-netbsd/iso.h
 
 isisd_SOURCES = \
-	isis_main.c $(libisis_a_SOURCES)
+	isis_main.c $(libisis_a_SOURCES) \
+	isis_bpf.c isis_dlpi.c isis_pfpacket.c
 
-isisd_LDADD = $(isis_method) @ISIS_TOPOLOGY_LIB@ ../lib/libzebra.la @LIBCAP@
-
-isisd_DEPENDENCIES = $(isis_method)
-
-EXTRA_DIST = isis_bpf.c isis_dlpi.c isis_pfpacket.c
+isisd_LDADD = @ISIS_TOPOLOGY_LIB@ ../lib/libzebra.la @LIBCAP@
 
 examplesdir = $(exampledir)
 dist_examples_DATA = isisd.conf.sample
diff --git a/isisd/isis_bpf.c b/isisd/isis_bpf.c
index 8c3602d..05f1138 100644
--- a/isisd/isis_bpf.c
+++ b/isisd/isis_bpf.c
@@ -21,6 +21,7 @@
  */
 
 #include <zebra.h>
+#if ISIS_METHOD == ISIS_METHOD_BPF
 #include <net/if.h>
 #include <netinet/if_ether.h>
 #include <sys/time.h>
@@ -339,3 +340,5 @@
 {
   return ISIS_OK;
 }
+
+#endif /* ISIS_METHOD == ISIS_METHOD_BPF */
diff --git a/isisd/isis_dlpi.c b/isisd/isis_dlpi.c
index 3cbe0b4..fe872a9 100644
--- a/isisd/isis_dlpi.c
+++ b/isisd/isis_dlpi.c
@@ -21,6 +21,7 @@
  */
 
 #include <zebra.h>
+#if ISIS_METHOD == ISIS_METHOD_DLPI
 #include <net/if.h>
 #include <netinet/if_ether.h>
 #include <sys/types.h>
@@ -622,3 +623,5 @@
     sock_buff, stream_get_endp (circuit->snd_stream) + LLC_LEN, 0);
   return ISIS_OK;
 }
+
+#endif /* ISIS_METHOD == ISIS_METHOD_DLPI */
diff --git a/isisd/isis_pfpacket.c b/isisd/isis_pfpacket.c
index 8752dba..9e4165e 100644
--- a/isisd/isis_pfpacket.c
+++ b/isisd/isis_pfpacket.c
@@ -21,6 +21,7 @@
  */
 
 #include <zebra.h>
+#if ISIS_METHOD == ISIS_METHOD_PFPACKET
 #include <net/ethernet.h>	/* the L2 protocols */
 #include <netpacket/packet.h>
 
@@ -371,3 +372,5 @@
 
   return ISIS_OK;
 }
+
+#endif /* ISIS_METHOD == ISIS_METHOD_PFPACKET */