aclocal: modernize and simplify AC_CONFIG_MACRO_DIR* tracing

aclocal: modernize and simplify AC_CONFIG_MACRO_DIR* tracing

Do so by assuming that AC_CONFIG_MACRO_DIRS is defined, and that we can
handle it and AC_CONFIG_MACRO_DIR by tracing AC_CONFIG_MACRO_DIR_TRACE.
We can do so since Automake 1.14 will require Autoconf 2.70 or later.

* Makefile.am (automake_internal_acdir): Remove.
(dist_automake_internal_ac_DATA): Likewise.
* m4/internal/ac-config-macro-dirs.m4: Delete.
* aclocal.in (trace_used_macros): No longer pass the path of
'internal/ac-config-macro-dirs.m4' to the autom4te invocation.
No longer trace nor handle macros 'AC_CONFIG_MACRO_DIR' and
'_AM_CONFIG_MACRO_DIRS'.
No longer uniquify '@ac_config_macro_dirs' explicitly.
($ac_config_macro_dirs_fallback): Remove.
No longer emit its content in the generated aclocal.m4.
* t/aclocal-acdir.sh: Remove a now-unneeded workaround.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>