bootstrap: fetch defaults from gnulib-cache.m4 for non-imported projects.

Recent changes uncovered some bugs in handling of gnulib-tool
options for projects that check in gnulib-cache.m4 rather than
keep a separate list of modules and options in bootstrap.conf.
* gl/build-aux/bootstrap.in (func_require_doc_base)
(func_require_gnulib_name, func_require_local_gl_dir): New
functions to set defaults from gnulib-cache.m4 in projects that
use gnulib-tool in --update mode.
(func_require_gnulib_tool_base_options): Use them to fetch
defaults.
(func_require_gnulib_copy_cmd): New function to calculate the
options required for gnlib-tool copy command.
(func_gnulib_tool_copy_file): Simplify accordingly.
(scriptversion): Bump.

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