don't show spurious error message when xcode-select is missing
don't show spurious error message when xcode-select is missing

Summary:
The build outputs on linux get polluted with

  /bin/sh: 1: xcode-select: not found
  ls: cannot access /Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator*.sdk: No such file or directory

messages

Test Plan: the messages are gone

Reviewers: mathieubaudet, akotulski

Reviewed By: akotulski

Subscribers: martinoluca

Differential Revision: https://phabricator.fb.com/D2341544

Tasks: 7824717

Signature: t1:2341544:1439471285:0c3584da043fe1edd486fc38917efa0732319f14

raphaelamorim/facebook-clang-plugins,raphaelamorim/facebook-clang-plugins,CJstar/facebook-clang-plugins,raphaelamorim/facebook-clang-plugins,raphaelamorim/facebook-clang-plugins,CJstar/facebook-clang-plugins,CJstar/facebook-clang-plugins,CJstar/facebook-clang-plugins