Add option to build and install APR automatically

[rm5248]

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.

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

[rm5248]

I messed around with this a little bit today without too much luck.

I have discovered the following:

• ExternalProject_Add won't work, because the libraries will not have been built yet
• FetchContent does do the build at configure time, but due to how APR/APR-Util are built they have a very old cmake configuration that doesn't seem to play nice at the moment. Specifically, APR-Util always tries to look in the install prefix(e.g. /usr/local/include) for APR by default.
• Expat does have a regularly updated CMake build, but I haven't gotten far enough to see if it works yet

It may be worth it to update the APR CMake build first, before attempting this.

Alternatively, we fork the relevant parts of APR into Log4cxx. I would prefer to move to standard C++ libraries(e.g. anything in namespace std) if possible, or Boost. While Boost can be somewhat annoying to build at times, it is widely used so it seems like a good compromise.