autotools: add automatic files generation

This patch modifies autogen.sh so that it is able to generate
config.sub, config.guess and ltmain.sh files which are not
anymore part of the git tree.

autogen.sh

@@ -1,4 +1,5 @@
 #!/bin/sh
 # Run this to generate all the initial makefiles, etc.
+libtoolize -c
 autoreconf -fv --install
 echo "You can now run \"./configure\" and then \"make\"."

libhtp/Makefile.am

@@ -1,3 +1,4 @@
+ACLOCAL_AMFLAGS = -I m4
 
 SUBDIRS= $(GENERIC_LIBRARY_NAME) test