[cmake] Treat polly as "in tree" if LLVM_EXTERNAL_POLLY_SOURCE_DIR is provided

Differential Revision: https://reviews.llvm.org/D25354

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@283608 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed