maint: simplify and improve safety of bootstrap process.

maint: simplify and improve safety of bootstrap process.

* Makefile.am (bootstrap_files): List files that need to be
generated at bootstrap time before `./configure && make' can
work.  It turns out that this is considerably fewer files than we
had thought necessary previously.
(bootstrap-deps-prep): Ensure minimum set of required substitution
variables are non-empty.
(bootstrap-deps): Depend on `bootstrap' files.
* bootstrap (Generate bootstrap dependencies): Now that
`Makefile.am' is entirely responsible for rebuilding files at
bootstrap time, we need only specify the new `bootstrap-deps'
target, and supply values for the substitutions checked by
`bootstrap-deps-prep'.
* configure.ac (AC_CONFIG_SRCDIR): `libtoolize.in' is not here yet
right after running `bootstrap'.  So rely on the presence of
`libltdl/config/libtoolize.m4sh', which is always there.

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