Don't hardcode urls when SCRIPT_NAME is available

Also, let the makefile define the name of the installed cgi and
use that definition as a default value for cgit_script_name variable.

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

cgit.h

@@ -75,6 +75,7 @@ extern char *cgit_logo;
 extern char *cgit_logo_link;
 extern char *cgit_module_link;
 extern char *cgit_virtual_root;
+extern char *cgit_script_name;
 extern char *cgit_cache_root;
 
 extern int cgit_nocache;