ICU-7747 CMake Port

[clemenswasser]

Checklist

• [x] Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-7747
• [x] Required: The PR title must be prefixed with a JIRA Issue number.
• [x] Required: The PR description must include the link to the Jira Issue, for example by completing the URL in the first checklist item
• [x] Required: Each commit message must be prefixed with a JIRA Issue number.
• [ ] Issue accepted (done by Technical Committee after discussion)
• [x] Tests included, if applicable
• [ ] API docs and/or User Guide docs changed or added, if applicable

Summary

This adds CMake and CTest support to ICU4C. I haven't yet tested this on all platforms and there is still some issue with the generation of icudt.dll, which causes a ICU Initialization returned: U_FILE_ACCESS_ERROR when running icuinfo and most tests, hence this is marked as a draft. Since I'm not so familiar with the icudt generation, I hope that you can help me with fixing this issue.