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.c

@@ -244,6 +244,8 @@ int main(int argc, const char **argv)
 	cgit_repolist.repos = NULL;
 
 	cgit_read_config(CGIT_CONFIG, cgit_global_config_cb);
+	if (getenv("SCRIPT_NAME"))
+		cgit_script_name = xstrdup(getenv("SCRIPT_NAME"));
 	if (getenv("QUERY_STRING"))
 		cgit_querystring = xstrdup(getenv("QUERY_STRING"));
 	cgit_parse_args(argc, argv);