add AC_CONFIG_MACRO_DIR([m4]), move *.m4 files to m4/

Silences notice on newer autoconf:

libtoolize: Consider adding `AC_CONFIG_MACRO_DIR([m4])' to configure.ac and
libtoolize: rerunning libtoolize, to keep the correct libtool macros in-tree.

This helps keep the top-level build directory clean, too

Makefile.am

@@ -1,4 +1,4 @@
-ACLOCAL_AMFLAGS = -I .
+ACLOCAL_AMFLAGS = -I m4
 
 # create default-config.h
 BUILT_SOURCES = default-config.h default-theme.h irssi-version.h

configure.ac

@@ -4,6 +4,7 @@ AC_CONFIG_AUX_DIR(build-aux)
 AC_PREREQ(2.50)
 
 AC_CONFIG_HEADERS([irssi-config.h])
+AC_CONFIG_MACRO_DIR([m4])
 AM_INIT_AUTOMAKE([1.9 no-define foreign])
 
 AM_MAINTAINER_MODE