Ship a Poco find script that supports checking versions

[gavanderhoorn]

The FindPoco.cmake provided by cmake_modules doesn't set the required variables for CMake to actually verify that it found the requested version of libpoco. At the moment the find_package(Poco .. 1.4.3 ..) is thus not enforced. This can lead to build issues on systems where libpoco-dev is installed, but not the right version.

This should be fixed by adapting FindPoco.cmake to also extract the version of the found libpoco-dev, and then shipping that version of the find script with the sources.

[gavanderhoorn]

The FindPoco.cmake in ros2/poco_vendor (this one) does some checking for versions, but does not appear to follow CMake's way of doing things (storing the version in a Poco_VERSION variable).

This could possibly be adapted.

[de-vri-es]

Note that poco ships its own PocoConfig.cmake, which is always preferable over a custom FindXXX.cmake. So there really should be no need for this.

/edit:
I just noticed that although PocoConfig.cmake is almost 5 years old [1], it does not appear in ubuntu 16.04 [2] :/ So nevermind I suppose.

[1] https://github.com/pocoproject/poco/commits/master/cmake/PocoConfig.cmake.in
[2] https://packages.ubuntu.com/xenial/amd64/libpoco-dev/filelist