Fix lldb build on older OSX versions after svn commit r244716

Older OSX versions don't define NSOperatingSystemVersion, so building
lldb gets: error: unknown type name 'NSOperatingSystemVersion'
This patch fixes the build by having GetOSVersionNumbers return false if
__ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__ < 101000, causing lldb to
behave the same as it did before the commit.

Reviewed by: jasonmolenda
Subscribers: lldb-commits
Differential Revision: http://reviews.llvm.org/D12396

llvm-svn: 246138