[cmake] Search for OpenMP from the package configuration file

[blattms]

We add target OpenMP::OpenMP_CXX to the linker line. Hence we always need to search for it, at least if the module is use by non-OPM modules.

Otherwise you might get the same errror as for the (autopkgtests in Debian](https://salsa.debian.org/science-team/opm-common/-/jobs/7439843):

-- No module specific tests for module 'opm-common' ('OpmCommonMacros.cmake' not found)
-- Setting opm-common_INCLUDE_DIRS=/usr/include;/usr/include/cjson
-- Setting opm-common_LIBRARIES=$<TARGET_FILE:opmcommon>;fmt::fmt;OpenMP::OpenMP_CXX;Boost::system;Boost::unit_test_framework;/usr/lib/x86_64-linux-gnu/libcjson.so
...
 * Vc, C++ Vectorization library, <https://github.com/VcDevel/Vc>
   For use of SIMD instructions
-- Configuring done (5.0s)
CMake Error at src/CMakeLists.txt:1 (add_executable):
  Target "dune-autopkgtest" links to:
    OpenMP::OpenMP_CXX
  but the target was not found.  Possible reasons include:
    * There is a typo in the target name.
    * A find_package call is missing for an IMPORTED target.
    * An ALIAS target is missing.

While this only ocurs if you use opm-common as a dune module, it still would be great to have this in the release. We do have people that are using opm-grid at least.

[blattms]

jenkins build this please

[akva2]

this isn't quite correct as it will override opm's ability to disable opemp. you'll have to carry a patch until i can look into a proper solution.

[blattms]

And this possibility to disable is used by jenkins for testing compilation?

[akva2]

nope, jenkins always builds with openmp, the stuff i am referring to is https://github.com/OPM/opm-common/blob/master/cmake/Modules/OpmLibMain.cmake#L61

[blattms]

Yeah, that simply means that nobody ever disables it. Even without the switch you can simply disable it by setting the number of threads to 1 at runtime, right?

Hence I don't even see a need to keep the switch.

[akva2]

uhm, that's a somewhat bold claim. i use it all the time while debugging. for instance, running in valgrind gomp causes a lot of analytics.

we can fix this while keeping the option intact. we just need to add the code to the config file instead of the prereqs, but right now there is no pretty mechanism to handle such things, we only have https://github.com/OPM/opm-common/blob/master/CMakeLists.txt#L32 which would mean we had to handle it in every module that uses openmp, a rather sub-optimal approach.