python: silence macro redefinition warnings

Python defines _XOPEN_SOURCE and _POSIX_C_SOURCE unconditionally.  On Linux,
this is problematic as glibc's features.h defines these values if _GNU_SOURCE is
defined to the value that is currently implemented.  Python defines it to the
versions that it requires, which may be different.  Undefine the macros on Linux
(technically, this should be safe to do globally) before including the python
headers.

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@208520 91177308-0d34-0410-b5e6-96231b3b80d8
Fix broken TestMultithreaded on Linux.

The clean line is failing under Ubuntu 12.04/gcc.  It cleans fine
without it on MacOSX.  It doesn't clean right on Linux but at least
now it doesn't fail the test.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@208713 91177308-0d34-0410-b5e6-96231b3b80d8