Fix building tests without system headers on Darwin

[beanz]

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".

[zturner]

If you wanted to get really fancy, you could actually locate the system include directory and set the default to that. But that's up to you, just an idea.

[tfiala]

(Retro) yep, this looked fine.

lldb/trunk/packages/Python/lldbsuite/test/dotest_args.py

group.add_argument(
         '--arch',
         metavar='arch',
         action='append',
         dest='archs',
         help=textwrap.dedent('''Specify the architecture(s) to test. This option can be specified more than once'''))
     group.add_argument('-C', '--compiler', metavar='compiler', dest='compilers', action='append', help=textwrap.dedent(
         '''Specify the compiler(s) used to build the inferior executables. The compiler path can be an executable basename or a full path to a compiler executable. This option can be specified multiple times.'''))
     if sys.platform == 'darwin':
-        group.add_argument('--apple-sdk', metavar='apple_sdk', dest='apple_sdk', help=textwrap.dedent(
+        group.add_argument('--apple-sdk', metavar='apple_sdk', dest='apple_sdk', default="macosx", help=textwrap.dedent(
             '''Specify the name of the Apple SDK (macosx, macosx.internal, iphoneos, iphoneos.internal, or path to SDK) and use the appropriate tools from that SDK's toolchain.'''))
     # FIXME? This won't work for different extra flags according to each arch.
     group.add_argument(
         '-E',
         metavar='extra-flags',
         help=textwrap.dedent('''Specify the extra flags to be passed to the toolchain when building the inferior programs to be debugged
                                                            suggestions: do not lump the "-A arch1 -A arch2" together such that the -E option applies to only one of the architectures'''))