**Description** Please explain the changes you made here.
If the feature changes current behaviour, explain why your solution is better.

:rotating_light: Before submitting your PR, please read [contribute](http://ghdl.readthedocs.io/en/latest/contribute.html#fork-modify-and-pull-request) in the [Docs](http://ghdl.readthedocs.io/en/latest/index.html), and review the following checklist:

- [ ] DO indicate which issues are either fixed or closed by this PR. See [GitHub Help: Closing issues using keywords](https://help.github.com/articles/closing-issues-via-commit-messages/).

**When contributing to the GHDL codebase...**

- [ ] DO make sure you are requesting to **pull a topic/feature/bugfix branch** (right side). Don't request your master!
- [ ] DO make sure you are making a pull request against the **master branch** (left side). Also you should start *your branch* off *our master*.
- [ ] DO make sure that GHDL can be successfully built. See [Building GHDL](https://github.com/ghdl/ghdl#building-ghdl).
- [ ] CONSIDER adding a unit test if your PR resolves an issue.
- [ ] CONSIDER modifying the docs, at least in the TODO, if your contribution is relevant to any of the content.
- [ ] AVOID breaking the continuous integration build.
- [ ] AVOID breaking the testsuite.

**When contributing to the docs...**

- [ ] DO make sure that the build is successful. See [ghdl/ghdl#572 (issuecomment-390466024)](https://github.com/ghdl/ghdl/issues/572#issuecomment-390466024).

**Further comments**

If this is a relatively large or complex change, kick off the discussion by explaining why you chose the solution you did, what alternatives you considered, etc.

:heart: Thank you!
