canonicalize: allow cross-testing from cygwin to mingw * modules/canonicalize-tests (configure.ac): Define HAVE_SYMLINK. (Makefile.am): Pass it through TESTS_ENVIRONMENT. * modules/canonicalize-lgpl-tests (configure.ac, Makefile.am): Likewise. * tests/test-canonicalize.sh: Also skip test if 'ln -s' works, but target does not support symlinks. * tests/test-canonicalize-lgpl.sh: Likewise. Signed-off-by: Eric Blake <ebb9@byu.net>