Fix for C++ runtime when OpenMP was not found (#10496)

Author: sjoelund

OMCompiler/SimulationRuntime/cpp/CMakeLists.txt

@@ -374,6 +374,7 @@ IF(OPENMP_FOUND)
 ELSE(OPENMP_FOUND)
   MESSAGE(STATUS "OPENMP disabled")
   SET(USE_OPENMP_ "OFF")
+  SET(OpenMP_CXX_FLAGS "")
 ENDIF(OPENMP_FOUND)
 
 # Find MPI