canonicalize: allow cross-testing from cygwin to mingw

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>