Added a layer of abstraction to make switching WPILibs easy.

Change-Id: Ic942ab6709df44e7807d661f2aa4cd44d43e69a7
diff --git a/aos/externals/BUILD b/aos/externals/BUILD
index 0991067..e42db6c 100644
--- a/aos/externals/BUILD
+++ b/aos/externals/BUILD
@@ -8,7 +8,7 @@
 ]
 
 cc_library(
-  name = 'wpilib',
+  name = 'wpilib_2015',
   visibility = ['//visibility:public'],
   srcs = glob([
     'allwpilib/wpilibc/wpilibC++/src/*.cpp',
@@ -35,4 +35,15 @@
     '-lspi',
     '-li2c',
   ],
+  defines = [
+    'WPILIB2015=1',
+  ],
+)
+
+cc_library(
+  name = 'wpilib',
+  visibility = ['//visibility:public'],
+  deps = [
+    ':wpilib_2015',
+  ],
 )