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

[echeran]

optional: I wonder if this gives the same result but might be more readable. (FWIW, I see similarly tricky preprocessor conditions being interspersed in lines 1171-1176.)

Suggested change

	#if (defined(CHECK_LOCALTIME_LINK) && !defined(DEBUG_SKIP_LOCALTIME_LINK)) && (U_PLATFORM == U_PF_LINUX)
	if (isValidOlsonID(tzid) && tzid[0] != '/')
	#endif
	return tzid;
	#if (defined(CHECK_LOCALTIME_LINK) && !defined(DEBUG_SKIP_LOCALTIME_LINK)) && (U_PLATFORM == U_PF_LINUX)
	/* Caller must handle threading issues */
	else if (gTimeZoneBufferPtr != nullptr ||
	extractOlsonIdFromPath(tzid, uprv_strlen(tzid))) {
	return gTimeZoneBufferPtr;
	}
	#endif
	#if (defined(CHECK_LOCALTIME_LINK) && !defined(DEBUG_SKIP_LOCALTIME_LINK)) && (U_PLATFORM == U_PF_LINUX)
	if (isValidOlsonID(tzid) && tzid[0] != '/') {
	return tzid;
	}
	/* Caller must handle threading issues */
	else if (gTimeZoneBufferPtr != nullptr ||
	extractOlsonIdFromPath(tzid, uprv_strlen(tzid))) {
	return gTimeZoneBufferPtr;
	}
	#else
	return tzid;
	#endif

[vlasky]

What's preventing this from being merged?