Add support for Eigen3 in Autotools compatibility layer
Don't search for disabled packages

Setting CMAKE_DISABLE_FIND_PACKAGE_xxx flag should make find_package
not doing anything, but this does not seem to work with older versions
of CMake. Anyway, we can test this flag and make sure that we don't
call find_package in that case.

Also includes a check for both upper- and same-case versions of found
flag, as some modules, most notably Boost, does not set the upper-case
version of their _FOUND flag; this tests for both.
Only require METIS if ALUGrid is built for parallel

If we have a parallel ALUGrid, then we must also link with METIS. If
METIS is not available, then parallel ALUGrid should be disabled. If
we have a serial-only ALUGrid, then METIS is not required.
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.