Add -I m4 back to ACLOCAL_AMFLAGS.

Add -I m4 back to ACLOCAL_AMFLAGS.

In commit b7b5d51, AC_CONFIG_MACRO_DIRS([m4]) was added to configure.ac,
which is the current proper way of including the m4 directory. However,
old versions of autoconf ignore it and need the -I m4 statement in
Makefile.am. Both the old and new way of indicating that the m4/
directory should be included can coexist.