Add option to build and install APR automatically

Since it can sometimes be hard to build APR, having a configuration option that will automatically build APR and expat as part of the log4cxx would be very useful.

This should be easily possible with [ExternalProject_Add](https://cmake.org/cmake/help/latest/module/ExternalProject.html).

By default, this option should be off and only opt-in.

Start with the repository's CMake configuration and the ExternalProject_Add documentation, then determine how APR and expat are currently located. Add an opt-in configuration option that builds and installs both dependencies, keeping it disabled by default. Done means the default build remains unchanged and the opt-in build handles APR and expat automatically.

Written by the indexing model from the issue text.