Issue a CMake warning if client project is incompatible

This puts a test into the ${project}-config.cmake file which warns
if the client project is built with a variable which is incompatible
with the build of opm-core.

Consider this minimal project:

    cmake_minimum_required (VERSION 2.8)
    set (HAVE_MPI 1)
    find_package (opm-core)

If linked with `-Dopm-core_DIR=` path to an opm-core tree which is
build *without* MPI-support, this will issue a warning when building
the client project.

This doesn't catch all cases (if a variable isn't defined, we cannot
know if it is an omission or if it has intentionally been left blank),
but at least it catches some.
Merge pull request #328 from rolk/328_warn

Issue a CMake warning if client project is incompatible