define constant only if it wasn't defined before
bug #12716 [ClassLoader] define constant only if it wasn't defined before (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
----------

[ClassLoader] define constant only if it wasn't defined before

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        |

Commits
-------

388229b define constant only if it wasn't defined before