ICU-13694 Use TZ=[:]<zonefile full path> in timezone detection on Linux+glibc

[jungshik]

Checklist

• [v] Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-13694
• [v] Required: The PR title must be prefixed with a JIRA Issue number.
• [v] Required: The PR description must include the link to the Jira Issue, for example by completing the URL in the first checklist item
• [v] Required: Each commit message must be prefixed with a JIRA Issue number.
• [v] Issue accepted (done by Technical Committee after discussion)
• [ ] Tests included, if applicable
• [v] API docs and/or User Guide docs changed or added, if applicable

[vlasky]

What's preventing this from being merged?