adopted OpenMP handling for libZFRelaxation.

src/external/libZFRelaxation/CMakeLists.txt

@@ -53,14 +53,10 @@ target_include_directories(
 )
 
 #--- add library dependencies -------------------------------------------------
+set(gomp "")
 if (OpenMP_FOUND)
-  target_compile_options(ZFRelaxation PUBLIC ${OpenMP_CXX_FLAGS})
+  set(gomp OpenMP::OpenMP_CXX)
 endif (OpenMP_FOUND)
-
-set(gomp "")
-if (OpenMP_FOUND AND (${CMAKE_HOST_SYSTEM_NAME} STREQUAL Linux))
-  set(gomp gomp)
-endif (OpenMP_FOUND AND (${CMAKE_HOST_SYSTEM_NAME} STREQUAL Linux))
 target_link_libraries(ZFRelaxation 
   ${gomp} FFTW3::FFTW3 ${ROOT_LIBRARIES} PUserFcnBase cuba BMWtools
 )