背景
昨天我构建了Caffe，它的依赖关系没有问题。
今天，由于找不到 protobuf 依赖项，我在构建 Caffe Matlab 包装器时遇到了问题。所以我重建了 Caffe:make clean后跟 a make all，希望能解决问题。
现在 Caffe 构建抱怨 protobuf 依赖。
错误输出在这个问题的底部给出。
在原始（成功）构建和失败构建之间，我需要pip install protobuf允许 Caffe python 包装器导入 protobuf，因为 python 抱怨无法找到 protobuf 包。这是 Caffe 重建失败之前涉及 protobuf 的唯一“更改”。
我曾尝试使用 brew 重新安装 protobuf，但这没有帮助。
所以基本上与 protobuf 相关的事件的年表如下：
1. protobuf installed from source using brew:
brew install --build-from-source --with-python --fresh -vd protobuf
2. caffe built successfully
3. caffe python wrappers failed to build -- protobuf not found
4. pip install protobuf
5. caffe python wrappers built successfully
6. caffe matlab wrappers failed to build -- protobuf not found
7. tried to rebuild caffe -- failed -- protobuf not found
8. reinstalled protobuf from source using brew
9. tried again to rebuild caffe -- failed -- protobuf not found
每当找不到 protobuf 时，brew list --versions显示 protobuf (2.6.1) 已安装。
问题
有人可以解释一下为什么在明确安装时找不到protobuf吗？
尤其令人困惑的是，它最初是被发现的（在最初的成功构建期间），而现在尽管采用了相同的方法，但它却没有被发现。
这是错误输出：
$ make clean
$ make all -j8
[... truncated ...]
CXX examples/mnist/convert_mnist_data.cpp
CXX examples/siamese/convert_mnist_siamese_data.cpp
CXX .build_release/src/caffe/proto/caffe.pb.cc
AR -o .build_release/lib/libcaffe.a
LD -o .build_release/lib/libcaffe.so
clang: warning: argument unused during compilation: '-pthread'
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/ranlib: file: .build_release/lib/libcaffe.a(cudnn_conv_layer.o) has no symbols
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/ranlib: file: .build_release/lib/libcaffe.a(cudnn_pooling_layer.o) has no symbols
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/ranlib: file: .build_release/lib/libcaffe.a(cudnn_relu_layer.o) has no symbols
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/ranlib: file: .build_release/lib/libcaffe.a(cudnn_sigmoid_layer.o) has no symbols
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/ranlib: file: .build_release/lib/libcaffe.a(cudnn_softmax_layer.o) has no symbols
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/ranlib: file: .build_release/lib/libcaffe.a(cudnn_tanh_layer.o) has no symbols
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/ranlib: file: .build_release/lib/libcaffe.a(cudnn.o) has no symbols
Undefined symbols for architecture x86_64:
"google::protobuf::io::CodedOutputStream::WriteStringWithSizeToArray(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, unsigned char*)", referenced from:
caffe::Datum::SerializeWithCachedSizesToArray(unsigned char*) const in caffe.pb.o
caffe::FillerParameter::SerializeWithCachedSizesToArray(unsigned char*) const in caffe.pb.o
caffe::NetParameter::SerializeWithCachedSizesToArray(unsigned char*) const in caffe.pb.o
caffe::SolverParameter::SerializeWithCachedSizesToArray(unsigned char*) const in caffe.pb.o
caffe::SolverState::SerializeWithCachedSizesToArray(unsigned char*) const in caffe.pb.o
caffe::NetState::SerializeWithCachedSizesToArray(unsigned char*) const in caffe.pb.o
caffe::NetStateRule::SerializeWithCachedSizesToArray(unsigned char*) const in caffe.pb.o
...
"google::protobuf::internal::empty_string_", referenced from:
caffe::Datum::SharedCtor() in caffe.pb.o
caffe::Datum::MergeFrom(caffe::Datum const&) in caffe.pb.o
caffe::Datum::~Datum() in caffe.pb.o
caffe::Datum::SharedDtor() in caffe.pb.o
caffe::Datum::Clear() in caffe.pb.o
caffe::Datum::MergePartialFromCodedStream(google::protobuf::io::CodedInputStream*) in caffe.pb.o
caffe::NetParameter::SharedCtor() in caffe.pb.o
...
"google::protobuf::internal::WireFormatLite::WriteBytesMaybeAliased(int, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, google::protobuf::io::CodedOutputStream*)", referenced from:
caffe::Datum::SerializeWithCachedSizes(google::protobuf::io::CodedOutputStream*) const in caffe.pb.o
"google::protobuf::internal::WireFormatLite::WriteStringMaybeAliased(int, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, google::protobuf::io::CodedOutputStream*)", referenced from:
caffe::FillerParameter::SerializeWithCachedSizes(google::protobuf::io::CodedOutputStream*) const in caffe.pb.o
caffe::NetParameter::SerializeWithCachedSizes(google::protobuf::io::CodedOutputStream*) const in caffe.pb.o
caffe::SolverParameter::SerializeWithCachedSizes(google::protobuf::io::CodedOutputStream*) const in caffe.pb.o
caffe::SolverState::SerializeWithCachedSizes(google::protobuf::io::CodedOutputStream*) const in caffe.pb.o
caffe::ParamSpec::SerializeWithCachedSizes(google::protobuf::io::CodedOutputStream*) const in caffe.pb.o
caffe::LayerParameter::SerializeWithCachedSizes(google::protobuf::io::CodedOutputStream*) const in caffe.pb.o
caffe::TransformationParameter::SerializeWithCachedSizes(google::protobuf::io::CodedOutputStream*) const in caffe.pb.o
...
"google::protobuf::internal::InitEmptyString()", referenced from:
caffe::Datum::SharedCtor() in caffe.pb.o
caffe::FillerParameter::SharedCtor() in caffe.pb.o
caffe::NetParameter::SharedCtor() in caffe.pb.o
caffe::SolverParameter::SharedCtor() in caffe.pb.o
caffe::SolverState::SharedCtor() in caffe.pb.o
caffe::NetState::SharedCtor() in caffe.pb.o
caffe::NetStateRule::SharedCtor() in caffe.pb.o
...
"google::protobuf::internal::empty_string_once_init_", referenced from:
caffe::Datum::SharedCtor() in caffe.pb.o
caffe::FillerParameter::SharedCtor() in caffe.pb.o
caffe::NetParameter::SharedCtor() in caffe.pb.o
caffe::SolverParameter::SharedCtor() in caffe.pb.o
caffe::SolverState::SharedCtor() in caffe.pb.o
caffe::NetState::SharedCtor() in caffe.pb.o
caffe::NetStateRule::SharedCtor() in caffe.pb.o
...
"google::protobuf::io::CodedInputStream::BytesUntilTotalBytesLimit() const", referenced from:
bool google::protobuf::internal::WireFormatLite::ReadPackedPrimitive<float, (google::protobuf::internal::WireFormatLite::FieldType)2>(google::protobuf::io::CodedInputStream*, google::protobuf::RepeatedField<float>*) in caffe.pb.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make: *** [.build_release/lib/libcaffe.so] Error 1