build: combine iptables-multi and iptables-static

Changed the Makefile so that:

1. --enable-shared / --disable-shared control the linkage against
   libdl (and thus the potential to use 3rd party extensions)

2. --enable-static / --disable-static controls whether shipped
   extensions are built-in or provided as modules

iptables-static becomes redundant by this action; iptables-multi now
has the feature.

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>
diff --git a/iptables-save.c b/iptables-save.c
index f63ee6b..3bcf422 100644
--- a/iptables-save.c
+++ b/iptables-save.c
@@ -148,7 +148,7 @@
 				iptables_globals.program_version);
 		exit(1);
 	}
-#ifdef NO_SHARED_LIBS
+#if defined(ALL_INCLUSIVE) || defined(NO_SHARED_LIBS)
 	init_extensions();
 #endif