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