Improve the way that VERSION_EXTRA is calculated

Instead of doing magic with the result from version.sh so that we can add a
preprocessor macro, the script now just writes src/version.cpp with the expected
content. This should fix all the various issues that we have with quoting the
arguments and things that go wrong when not building znc from a git clone.

Signed-off-by: Uli Schlachter <psychon@znc.in>