bootstrap: always extract only the first AC_CONFIG_MACRO_DIR arg.

bootstrap: always extract only the first AC_CONFIG_MACRO_DIR arg.

Previous versions of bootstrap concatenated all the arguments to
multiple invocations of AC_CONFIG_MACRO_DIR when extracting the
macro_dir directory name.  This change enforces correct and
consistent behaviour.
* libtoolize.in (func_require_macro_dir): Use the new
func_extract_trace_first function to make sure the first argument
is always used.
(func_require_aux_dir, func_require_gnulib_mk)
(func_require_source_base): Future proof these functions against
multiple invocations or additional arguments to the macros they
trace.

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