Fix building tests without system headers on Darwin

Summary: Default installations of OS X do not have system headers installed at /usr/include. This patch allows the LLDB test executables to properly compile when built on a system without headers at /usr/include by specifying a default value for the apple-sdk flag as "macosx".

Reviewers: tfiala, zturner

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D25487

llvm-svn: 284042