aclocal: smash newlines in arguments of traced macros

aclocal: smash newlines in arguments of traced macros

This change fixes the existing issues with AC_CONFIG_MACRO_DIRS
containing newlines:
<http://lists.gnu.org/archive/html/autoconf-patches/2012-11/msg00000.html>

Likely, it will also allow a less involved implementation of the
AM_EXTRA_RECURSIVE_TARGETS macro (but that is left for potential
follow-up patches).

* aclocal.in (trace_used_macros): When calling autom4te, pass its
'--trace' option an argument containing '${::}%' rather than '$1'.
According to the autoconf manual (as of version 2.69), that will expand
to the concatenation, with the '::' string, of all the arguments passed
to a macro, with all newline characters in such arguments smashed.
Related adjustments when handling the macro AC_CONFIG_MACRO_DIRS, to
ensure leading whitespace in its argument are handled correctly.
* t/aclocal-macrodirs.tap ("AC_CONFIG_MACRO_DIRS: extra whitespace"):
No longer declare it as an xfailing test.

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