Fix icu usage in builds

The icu build time path patching resulted in corrupted lines
e.g. in qconfig.pri.

Prefer CPATH environment variable over -I (for configure).
Also do not use -L configure option for icu libs.

With this change there should not be any icu build time paths
in files thus the icu build time path patching is not
performed.

Task-number: QTBUG-42599
Change-Id: I1956c6d0ec642c0df757ac8467e2867711a084a1
Reviewed-by: Kai Koehne <kai.koehne@theqtcompany.com>