Pacify GCC. Some of these problems were reported by Bruno Haible in:
https://lists.gnu.org/r/bug-gnulib/2020-12/msg00217.html
* lib/canonicalize-lgpl.c, lib/canonicalize.c:
Sort shared include directives, for consistency.
(IF_LINT): New macro.
(suffix_requires_dir_check): Mark with _GL_ATTRIBUTE_PURE.
* lib/canonicalize-lgpl.c (GCC_LINT, _GL_ATTRIBUTE_PURE) [_LIBC]:
New macros.
(realpath_stk): Suppress bogus -Wmaybe-uninitialized warning.
* lib/canonicalize.c (canonicalize_filename_mode_stk):
Omit unused local. Suppress bogus -Wmaybe-uninitialized warning.