syntax-check: fix violations and re-enable sc_prohibit_strcmp.

syntax-check: fix violations and re-enable sc_prohibit_strcmp.

* cfg.mk (local-checks-to-fix): Remove sc_prohibit_strcmp
from list of disabled checks.
(exclude_file_name_regexp--sc_prohibit_strcmp): Ignore
violations in libtool.texi and any demo C files.
* libltdl/libltdl/lt__private.h (strneq, streq): Renamed from
this...
(STRNEQ, STREQ): ..to this.  Adjust all callers.
* tests/slist.at: Add STREQ definition. Change all !strcmp calls
to STREQ.
* build-aux/ltmain.m4sh (func_emit_cwrapperexe_src): Add and use
STREQ definition.

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