fix bug of "-l-ldeepmd"

Author: Lu

source/CMakeLists.txt

@@ -173,7 +173,7 @@ if (BUILD_CPP_IF)
   if (USE_CUDA_TOOLKIT)
     set (LIB_DEEPMD_OP_CUDA		"deepmd_op_cuda")
   else()
-    set (LIB_DEEPMD_OP_CUDA		"")
+    set (LIB_DEEPMD_OP_CUDA		"deepmd_op")
   endif()
   if (CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 4.9)
     set (LIB_DEEPMD_NATIVE	"deepmd_native_md")