Change network dependence on configure / build time

[mwtoews]

This item is to address some points raised by @gdt starting in this mail list thread, around "it's a bug to try to use the network at build time".

I have some work-in-progress to address this item (thus I'm assigned), which I hope to put out very soon, as I'm aware there could be a RC in the coming week.

My proposed solution is to have a CMake option USE_NETWORK with default OFF. It will show warnings if test dependencies cannot be met, and run a subset of tests that can still run.