fix building when USE=-curl is specified (bug #572074)

--- ./code/client/cl_main.c.orig
+++ ./code/client/cl_main.c
@@ -1872,7 +1872,11 @@
 }
 
 qboolean CL_IsDownloading(void) {
+#if USE_CURL
 	return clc.cURLUsed;
+#else
+	return qfalse;
+#endif
 }
 
 /*