Fix compilation directive for PPC.

Fix compilation for iOS on XCode 4.5

 Compilation directive for PPC was using MAC_OS_X_VERSION_MIN_REQUIRED.

 This is not correct, as the latest SDKs allow to compile for older version of
Mac OS, but don't contain the ppc headers. Changing the directive to use
MAC_OS_X_VERSION_MAX_ALLOWED instead.

 Moreover, uploader.mm was including pwd.h that was not used and doesn't exist
on the latest iOS SDK.
Committed: https://code.google.com/p/google-breakpad/source/detail?r=982

[Mark Mentovai]

LGTM