tests: always extract only the first AC_CONFIG_MACRO_DIR arg.

tests: always extract only the first AC_CONFIG_MACRO_DIR arg.

Previous releases of libtoolize used the final invocation when
there were several, and after the rewrite over extract-trace,
all the invocation directories were concatenated.  This change
enforces correct and consistent behaviour.
* tests/libtoolize.at (multiple AC_CONFIG_MACRO_DIR invocation):
New test.
* build-aux/extract-trace (func_extract_trace_first): New function
for clients that source this file, which returns only the first
argument to the first invocation of the named macros.
* libtoolize.in (func_require_ac_macro_dir): Use it to make sure
the first argument is always used.
(func_require_ac_aux_dir, func_require_ac_ltdl_dir)
(func_require_ac_ltdl_options): Future proof these functions against
multiple invocations or additional arguments to the macros they
trace.
* News (Important incompatible changes): Note change in semantics.
Reported by Eric Blake.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>