fix openmp not found

CMakeLists.txt

@@ -27,8 +27,10 @@ option(NCNN_OPENCV "minimal opencv structure emulation" OFF)
 
 if(NCNN_OPENMP)
     find_package(OpenMP)
-    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${OpenMP_C_FLAGS}")
-    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${OpenMP_CXX_FLAGS}")
+    if(OpenMP_CXX_FOUND OR OPENMP_FOUND)
+        set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${OpenMP_C_FLAGS}")
+        set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${OpenMP_CXX_FLAGS}")
+    endif()
 endif()
 
 add_definitions(-Wall -Wextra)