Fix: add OpenMP flag properly.

Fixes #945.
Related: #964

Author: caic99

CMakeLists.txt

@@ -53,6 +53,8 @@ target_link_libraries(${ABACUS_BIN_NAME} Threads::Threads)
 if(USE_OPENMP)
   find_package(OpenMP REQUIRED)
   target_link_libraries(${ABACUS_BIN_NAME} OpenMP::OpenMP_CXX)
+  add_compile_options(${OpenMP_CXX_FLAGS})
+  add_link_options(${OpenMP_CXX_LIBRARIES})
 endif()
 
 include(CheckLanguage)