modernize the way how to check/deal with OpenMP. This helps also on the macOS side.

CMakeLists.txt

@@ -110,13 +110,13 @@ find_package(FFTW3 REQUIRED)
 find_package(LibXml2 REQUIRED)
 
 #--- check for OpenMP ---------------------------------------------------------
-if (try_OpenMP AND NOT APPLE)
+if (try_OpenMP)
   find_package(OpenMP)
-  if (OpenMP_FOUND)
+  if (OpenMP_CXX_FOUND)
     add_definitions(-DHAVE_GOMP)
     set(HAVE_GOMP 1 CACHE INTERNAL "Have GOMP")
-  endif (OpenMP_FOUND)
-endif (try_OpenMP AND NOT APPLE)
+  endif (OpenMP_CXX_FOUND)
+endif (try_OpenMP)
 
 #--- check for CUDA and/or OpenCL followed by DKS -----------------------------
 if (dks)