Reinstate code to embed the svn version (or git commit id) in the
output given by varnishd -V.

Also, make sure we don't overwrite version.c when the tree is
exported, but generate it if it's completely missing.  (This should
only happen if somebody does svn export rather than make dist, but
some people might do that.)

git-svn-id: http://www.varnish-cache.org/svn/trunk@4175 d4fa192b-c00b-0410-8231-f00ffab90ce4