aclocal: fix for more-than-once specified directories

aclocal: fix for more-than-once specified directories

Related to automake bug#13514.

Do not consider directories for extra m4 files multiple times in
'aclocal'.  Doing so caused problems on older packages that specify

    configure.ac:  AC_CONFIG_MACRO_DIRS([m4])
    Makefile.am:   ACLOCAL_AMFLAGS = -I m4

if the 'm4' directory does not exist when aclocal is called the first
time by autoreconf.

See:
<http://lists.gnu.org/archive/html/bug-automake/2013-01/msg00115.html>

* aclocal.in (scan_m4_files): Remove duplicates in @user_includes.
* t/aclocal-macrodir.tap: Extend.
* t/aclocal-macrodirs.tap: Likewise.

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