We only support building on Linux, so do not run cobaltb.py otherwise.

Change-Id: Ie983fd1e3e8406b65552bfe5dc706282dbd8aff5
diff --git a/cobaltb.py b/cobaltb.py
index 282a9d7..991e5e9 100755
--- a/cobaltb.py
+++ b/cobaltb.py
@@ -972,6 +972,9 @@
     os.chdir(savedDir)
 
 def main():
+  if not sys.platform.startswith('linux'):
+    print 'Only linux is supported!'
+    return 1
   # We parse the command line flags twice. The first time we are looking
   # only for two particular flags, namely --production_dir and
   # --cobalt_on_personal_cluster. This first pass