Gpu is forced? make with cpu_only = 1 failed.

[jingmouren]

I think running demo with pretrained model don't need a GPU.
So I change the Makefile.config with cpu_only := 1
but the following erroe occurs when make:

../lib/libcaffe.a(layer_factory.cpp.o):(.data.rel.ro._ZTVN5caffe16ConvolutionLayerIfEE[_ZTVN5caffe16ConvolutionLayerIfEE]+0xd0)：对‘caffe::ConvolutionLayer::RotateARF_gpu()’未定义的引用
../lib/libcaffe.a(layer_factory.cpp.o):(.data.rel.ro._ZTVN5caffe16ConvolutionLayerIfEE[_ZTVN5caffe16ConvolutionLayerIfEE]+0xe0)：对‘caffe::ConvolutionLayer::AlignARF_gpu()’未定义的引用
../lib/libcaffe.a(layer_factory.cpp.o):(.data.rel.ro._ZTVN5caffe16ConvolutionLayerIdEE[_ZTVN5caffe16ConvolutionLayerIdEE]+0xd0)：对‘caffe::ConvolutionLayer::RotateARF_gpu()’未定义的引用
../lib/libcaffe.a(layer_factory.cpp.o):(.data.rel.ro._ZTVN5caffe16ConvolutionLayerIdEE[_ZTVN5caffe16ConvolutionLayerIdEE]+0xe0)：对‘caffe::ConvolutionLayer::AlignARF_gpu()’未定义的引用
collect2: error: ld returned 1 exit status
tools/CMakeFiles/convert_imageset.dir/build.make:130: recipe for target 'tools/convert_imageset' failed
make[2]: *** [tools/convert_imageset] Error 1
CMakeFiles/Makefile2:441: recipe for target 'tools/CMakeFiles/convert_imageset.dir/all' failed
make[1]: *** [tools/CMakeFiles/convert_imageset.dir/all] Error 2
Makefile:129: recipe for target 'all' failed
make: *** [all] Error 2