Add setting to enable/disable extra links on index page

The summary/log/tree links displayed for each repository on the index
page lost some of their purpose when the header menu was added, so this
commit introduces the parameter 'enable-index-links' which must be set
to 1 to enable these links.

Suggested-by: Kristian Høgsberg <krh@bitplanet.net>
Signed-off-by: Lars Hjemli <hjemli@gmail.com>

cgit.h

@@ -118,6 +118,7 @@ extern char *cgit_repo_group;
 
 extern int cgit_nocache;
 extern int cgit_snapshots;
+extern int cgit_enable_index_links;
 extern int cgit_enable_log_filecount;
 extern int cgit_enable_log_linecount;
 extern int cgit_max_lock_attempts;