Ert cleanup

This change-set catches up to recent changes in Ensembles/ert (specifically commit Ensembles/ert@ca2665b).  We initially import commit e4866c67, but need to take a few additional steps to ensure that the branch can be built with ERT support.

Specifically,
* Update configure-time detection macro in `m4/ert.m4` to changing location and library names
* Update test program `tests/test_ert.cpp` for changing locations of ERT headers
* Update example utility `examples/import_rewrite.cpp` for changing location of ERT headers