tools/version.h: If the version is not available, use 0.0.0

It's useful when you only have shallow git history handy.
(eg. to save some network bandwidth)

The default value here was chosen to mirror the default of
tools/Makefile.win.

tools/version.sh

@@ -84,10 +84,8 @@ fi
 # Make sure we know what is going on
 
 if [ -z ${VERSION} ] ; then
-  echo "Missing versioning information"
-  echo $USAGE
-  echo $ADVICE
-  exit 1
+  echo "Missing versioning information. Using the dummy value. (0.0.0)"
+  VERSION="0.0.0"
 fi
 
 if [ -z ${OUTFILE} ] ; then