libtoolize: refactor copying filter creation.

libtoolize: refactor copying filter creation.

Prevent the various require_filter functions from becoming too
long by factoring common code into new functions.
* libtoolize (func_make_relative_dir_filter)
(func_make_relative_ltdl_filter): New functions. Generalize and
consolidate the various similar snippets for concatenating
together the sed scripts for func_copy filtering.
(require_filter_Makefile_am, require_filter_aclocal_m4)
(require_filter_configure_ac): Simplify.
(require_ltdl_relative_aux_dir): Remove.
(require_ltdl_relative_macro_dir): Ditto.

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