build: Possibly add verbose build option to ns3

Waf had two levels of build verbosity so that we could examine the flags used to build and link things.

./waf build -v
or
./waf build -vvv

I don't see a similar option in ns3. I don't know how hard it is to add this, given that cmake can use different build tools that may have different verbosity arguments.

I was able to work around this by using

./ns3 configure ...
cd cmake-cache
make VERBOSE=1

so perhaps since this is not frequently used, it may be suitable to just document how to do this for both ninja and make for the times that users may need to look at how CMake is building and linking things. Comments?