undefined reference to Poco::DigestEngine

first, `git clone https://github.com/conan-io/examples.git`
and then cd into `libraries/poco/md5`
run `./build.sh`

got error like that:

```
+ cmake --build .
Scanning dependencies of target md5
[ 50%] Building CXX object CMakeFiles/md5.dir/md5.cpp.o
[100%] Linking CXX executable bin/md5
CMakeFiles/md5.dir/md5.cpp.o: In function `main':
```
md5.cpp:(.text.startup+0x9b): undefined reference to `Poco::DigestEngine::digestToHex[abi:cxx11](std::vector > const&)'
collect2: error: ld returned 1 exit status
CMakeFiles/md5.dir/build.make:83: recipe for target 'bin/md5' failed
make[2]: *** [bin/md5] Error 1
CMakeFiles/Makefile2:72: recipe for target 'CMakeFiles/md5.dir/all' failed
make[1]: *** [CMakeFiles/md5.dir/all] Error 2
Makefile:83: recipe for target 'all' failed
make: *** [all] Error 2
```

Reproduce the failure by cloning the repository, entering libraries/poco/md5, and running ./build.sh. Inspect md5.cpp and the CMake build configuration around the failed link step; done means the bin/md5 executable builds successfully without the undefined Poco::DigestEngine reference.

Written by the indexing model from the issue text.