Used external roborio compiler instead of local compiler.

Change-Id: Ifec0d68b0b51a822c906bc4f52320004ee2f5d84
diff --git a/y2015/util/BUILD b/y2015/util/BUILD
index 27c144d..e520d5d 100644
--- a/y2015/util/BUILD
+++ b/y2015/util/BUILD
@@ -5,7 +5,7 @@
   deps = [
     '//third_party/eigen',
     '//y2015:constants',
-    '//external:libm',
+    '//debian:libm',
   ],
 )