v2: incorporate Roland's nitpicks (I hope)
v3: It was decided that it's better to change the default behavior
v4: make sure the CMake syntax is correct even if the value of the
variable contains spaces (if it contains double quotes it's a
different matter, but that's pretty much a corner case, IMHO)
v5: properly escape backslash and double quote characters in the cmake
syntax
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.
