use pure config mode.
we no longer compile a test application for the libraries,
however checks have been tightened by making sure version of
all opm modules are the same.
this allows for putting prereqs directly in the module repositories
As opm-common is search in the toplevel CMakeLists.txt of each module in
config mode and the dependencies and defines are only processed in
opm_find_package(opm-common), we allow a second search using module mode
to trigger opm_find_package.
Otherwise some defines and macros will not be set as this is done
in opm_find_package. This might also make sense for dunecontrol
which always will set *_DIR.
Even though people are telling that ecl is already found using
sibling search it turned out that this statement is false. It
was always found using the CMake package cache. When installing
this lead to the installed package using ecl from the build tree.
With this commit we first try to find ecl without the package cache
but maybe using sibling search. Building installed packages no works
by setting -DSIBLING_SEARCH=Off -DCMAKE_INSTALL_PREFIX=/install/path
