syntax-check: fix violations and implement sc_useless_braces_in_variable_derefs.

syntax-check: fix violations and implement sc_useless_braces_in_variable_derefs.

Until now, libtool sources have used braced variable names
seemingly at random! Almost always the braces are just noise, so
remove all the unnecessary ones.
* cfg.mk (sc_useless_braces_in_variable_derefs): New syntax
check rule to ensure we only reintroduce braced variable
dereferences if they are followed by a valid variable name
character.
build-aux/general.m4sh, build-aux/git-hooks/commit-msg,
build-aux/ltmain.m4sh, build-aux/options-parser, configure.ac,
libltdl/configure.ac, m4/libtool.m4, m4/ltdl.m4,
m4/ltoptions.m4, tests/defs.m4sh, tests/demo-nopic.test,
tests/depdemo/configure.ac, tests/flags.at, tests/link.test,
tests/objectlist.test, tests/quote.test, tests/static.at: Remove
spurious braces.

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