cmake: Fix building of Polly on Apple system

The Apple linker fails by default, if some function calls can not be resolved at
link time. However, all functions that are part of LLVM itself will not be
linked into Polly, but will be provided by the compiler that Polly is loaded
into.  Hence, during linking we need to ignore failures due to unresolved
function calls.

llvm-svn: 161234