avoid spurious test failure when library is built without ACL support
* m4/acl.m4 (USE_ACL): Define as a shell variable, too, for...
* modules/acl-tests (Makefile.am) [TESTS_ENVIRONMENT]: Add USE_ACL.
* tests/test-file-has-acl.sh: Skip if USE_ACL == 0.
* tests/test-copy-acl.sh: Likewise.