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>

cgit.h

@@ -98,7 +98,7 @@ struct taginfo {
 	char *msg;
 };
 
-extern const char cgit_version[];
+extern const char *cgit_version;
 
 extern struct repolist cgit_repolist;
 extern struct repoinfo *cgit_repo;