canonicalize, canonicalize-lgpl: honor // if distinct from / * modules/canonicalize (Files): Add double-slash-root.m4. * modules/canonicalize-lgpl (Files): Likewise. * m4/canonicalize.m4 (gl_FUNC_CANONICALIZE_FILENAME_MODE) (gl_CANONICALIZE_LGPL_SEPARATE): Add dependency. * lib/canonicalize.c (DOUBLE_SLASH_IS_DISTINCT_ROOT): Provide fallback definition. (canonicalize_filename_mode): Use it to protect //. * lib/canonicalize-lgpl.c (DOUBLE_SLASH_IS_DISTINCT_ROOT) (__realpath): Likewise. * tests/test-canonicalize.c (main): Test this. * tests/test-canonicalize-lgpl.c (main): Likewise. * modules/canonicalize-tests (Depends-on): Add same-inode. * modules/canonicalize-lgpl-tests (Depends-on): Likewise. Signed-off-by: Eric Blake <ebb9@byu.net>