[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.

Please backport to 3.2.

fixes #875 (closed)

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)