Missing `-lpthread` causes gcc test failure.

Summary:
`-lpthread` was removed for `g++ -std=c++11` on Linux due to llvm.org/pr21553,
but the original issue doesn't seem to be reproducible anymore. Even if it were,
it had nothing to do with compiling inferiors.

Reviewers: sbest, sivachandra, chying

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D8991

llvm-svn: 234770