Add version info from git-describe

A new script, gen-version.sh, is now invoked from 'make version' to generate
the file VERSION. This file contains a version identifier generated by
git-describe and is included in the Makefile.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>

.gitignore

@@ -1,4 +1,5 @@
 # Files I don't care to see in git-status/commit
 cgit
+VERSION
 *.o
 *~