OpenMP required (but still not used) for compilation.

CMakeLists.txt

@@ -35,6 +35,13 @@ if(COMPILER_SUPPORTS_CXX20)
 else()
   message(FATAL_ERROR "The compiler ${CMAKE_CXX_COMPILER} has no C++20 support. Please use a different C++ compiler.")
 endif()
+# OpenMP is required
+CHECK_CXX_COMPILER_FLAG(-fopenmp COMPILER_SUPPORTS_OPENMP)
+if(COMPILER_SUPPORTS_OPENMP)
+ set(default_options ${default_options} -fopenmp)
+else()
+  message(FATAL_ERROR "The compiler ${CMAKE_CXX_COMPILER} has no OpenMP support. Please use a different C++ compiler.")
+endif()
 # Link time optimization check
 set(CMAKE_REQUIRED_FLAGS -flto)
 CHECK_CXX_COMPILER_FLAG(-flto COMPILER_SUPPORTS_FLTO)