[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

Author: etherzhhb

CMakeLists.txt

@@ -395,6 +395,8 @@ option(LLVM_POLLY_BUILD "Build LLVM with Polly" ON)
 
 if (EXISTS ${LLVM_MAIN_SRC_DIR}/tools/polly/CMakeLists.txt)
   set(POLLY_IN_TREE TRUE)
+elseif(LLVM_EXTERNAL_POLLY_SOURCE_DIR)
+  set(POLLY_IN_TREE TRUE)
 else()
   set(POLLY_IN_TREE FALSE)
 endif()