syntax-check: fix violations and implement sc_useless_quotes_in_assignment.

syntax-check: fix violations and implement sc_useless_quotes_in_assignment.

Contrary to popular belief, Bourne shell does not resplit RHS of
assignments after expansion, so if there are no unquoted shell
metacharacters or whitespace, the quotes are useless.
* cfg.mk (_sc_search_regexp_or_exclude): A variation of gnulib's
_sc_search_regexp that also allows individual exclusions at the
site of what would otherwise match by adding a comment.
(sc_useless_quotes_in_assignment): New syntax-check rule to
ensure we don't reintroduce useless quoted assignments.
* bootstrap, bootstrap.conf, build-aux/edit-readme-alpha,
build-aux/general.m4sh, build-aux/getopt.m4sh,
build-aux/ltmain.m4sh, build-aux/options-parser, configure.ac,
doc/libtool.texi, libtoolize.m4sh, m4/libtool.m4, m4/ltdl.m4,
m4/ltoptions.m4, tests/cwrapper.at, tests/defs.m4sh,
tests/lalib-syntax.at, tests/libtoolize.at,
tests/lt_dlopenext.at, tests/mdemo/configure.ac,
tests/mdemo2/configure.ac, tests/need_lib_prefix.at,
tests/no-executables.at, tests/quote.test, tests/suffix.test:
Remove spurious quotes.

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