Binary stability

[mjvankampen]

The guys from conan raised a good issue that the cmake file of protolib now really depends on what you have installed on your system. This makes it hard to have predictable binaries. Do you think it is ok to make things like pcap and xml2 settings that you turn on instead of having them there 'by accident'.

Ref. https://github.com/conan-io/conan-center-index/pull/4299

[bebopagogo]

So, essentially instead of finding these automatically and building them in when available to allow explicit control of the build. I think that's OK, particularly since at the moment protolib is more of a set of C++ classes that can be included in a project rather than library. And then those features can be explicitly included as needed by the parent project via their CMake config? I am not yet sufficiently familiar with CMake to know exactly how, but I assume a parent CMakeList can set build options in "child" CMakeLists?