define ACLOCAL_AMFLAGS to include "m4" directory

$ autoreconf  --install --force
This command will search the "m4" files from different dirs.
The result is listed in autom4te.cache/requests.

Define m4 dir in Makefile.am:
ACLOCAL_AMFLAGS = -I m4

and define m4 dir in configure.in:
AC_CONFIG_MACRO_DIR([m4])

then the m4 macro defined in "m4" dir will be expanded correctly.

Signed-off-by: Kui Shi <skuicloud@gmail.com>