build: enable automake for extensions/ directory

Because we are likely to be having more userspace programs soon, and
reproducing manual makefiles is a bad idea, make extensions/ ready
for automake traversal. The build pattern now is:

1.  toplevel Makefile.am starts off with extensions/Makefile.am

2.  Makefile.am

    a.  builds programs in current directory

    b.  runs Kbuild
        - only from extensions/Makefile.am
          (so it does not get reinvoked from case 2cII)
        - Kbuild recurses on its own

    c.  runs Mbuild
        I.  builds iptables shared libraries in current directory
        II. runs Makefile.am in each subdir (goto step 2.)

INSTALL

@@ -46,6 +46,9 @@ Configuring and compiling
 	/lib/modules/$(running version)/build, which usually points to
 	the right directory. (If not, you need to install something.)
 
+	For RPM building, it should be /usr/src/linux-obj/...
+	or whatever location the distro makes use of.
+
 --with-xtables=
 
 	Specifies the path to the directory where we may find
@@ -55,11 +58,11 @@ Configuring and compiling
 	include/xtables.h. (The latter to support both standard
 	/usr/include and the iptables source root.)
 
---with-libxtdir=
+--with-xtlibdir=
 
 	Specifies the path to where the newly built extensions should
 	be installed when `make install` is run. It uses the same
-	default as the Xtables package, ${libexecdir}/xtables.
+	default as the Xtables/iptables package, ${libexecdir}/xtables.
 
 If you want to enable debugging, use
 
@@ -72,15 +75,10 @@ much easier.)
 Build-time options
 ==================
 
-V= controls the kernel's make verbosity.
+V= controls the verbosity of make commands.
 V=0	"silent" (output filename)
 V=1	"verbose" (entire gcc command line)
 
-VU= controls the Xt-a make verbosity.
-VU=0	output filename
-VU=1	output filename and source file
-VU=2	entire gcc command line
-
 
 Note to distribution packagers
 ==============================