build: avoid some warnings

* gl/lib/mbsalign.c (mbsalign): Mark unused parameter.
* bootstrap.conf (gnulib_modules): Remove obsolete
rename-dest-slash.
* gnulib-tests/Makefile.am (AM_CFLAGS): Reduce set of warnings for
gnulib tests.
* gl/modules/rename-tests.diff (Makefile.am): New file, to add
LIBINTL to LDADD, since we avoid canonicalize-lgpl module.
* gl/lib/regcomp.c.diff (regerror, calc_next)
(build_collating_symbol, parse_bracket_element, build_equiv_class)
(free_tree): Mark unused parameters.
* gl/lib/regex_internal.h.diff (re_string_elem_size_at): New file,
to mark unused parameters.
* gl/lib/printf-args.c.diff (PRINTF_FETCHARGS): New file, to avoid
type mismatch.
* gl/lib/vasnprintf.c (VASNPRINTF): New file, to avoid shadowing
local variable name.
* .gitignore: Ignore temporary build artifacts.

gl/modules/rename-tests.diff

@@ -0,0 +1,9 @@
+diff --git a/modules/rename-tests b/modules/rename-tests
+index be1b423..fea330a 100644
+--- a/modules/rename-tests
++++ b/modules/rename-tests
+@@ -14,3 +14,4 @@ configure.ac:
+ Makefile.am:
+ TESTS += test-rename
+ check_PROGRAMS += test-rename
++test_rename_LDADD = $(LDADD) @LIBINTL@