I've been building C::B from source for over a year now and about 9 times out of 10, the svn number on the splash and help menu does update. It's just that weird once in a while that it doesn't. My build process has always been the same.
Linux:
After the first time building it, to update I just do the following
svn update
make -j4
sudo make install
Windows:
Right click folder, svn update.
Open .cpb project, then build.
Open contrib plugins workspace, then build workspace.
Run update.bat.
Copy contents of output folder to where my "installation" of C::B is.
As I said above, it does update 90% of the time. Instructions in the wiki for how to do the initial build are more or less clear, but for updating there aren't really any clear steps, so it took some trial and error for my first times doing updates on each architecture. Never occurred to me to just delete autorevision.h and rebuild, thanks.