[opm][cmake] adapt to changes in OPM

It is currently required that CMake looks for OpenMP, see
https://github.com/OPM/opm-common/issues/1751.
On the other, setting `-fopenmp` in the test is not
necessary anymore.

cmake/modules/DumuxMacros.cmake

@@ -14,3 +14,7 @@ find_package(PTScotch)
 find_package(PVPython)
 find_package(Valgrind)
 find_package(Quadmath)
+# The following is required for being able to depend on OPM.
+# Remove once a better solution has been found at
+# https://github.com/OPM/opm-common/issues/1751.
+find_package(OpenMP)

test/porousmediumflow/2p/implicit/cornerpoint/CMakeLists.txt

@@ -6,7 +6,6 @@ dumux_add_test(NAME test_2p_cornerpoint
               SOURCES main.cc
               CMAKE_GUARD HAVE_OPM_GRID
               COMPILE_DEFINITIONS HAVE_ECL_INPUT=1
-              LINK_LIBRARIES -fopenmp
               COMMAND ${CMAKE_SOURCE_DIR}/bin/testing/runtest.py
               CMD_ARGS --script fuzzy
                        --files ${CMAKE_SOURCE_DIR}/test/references/test_2p_cornerpoint-reference.vtu