[Polly][CMake] Fix lit setup for building the in the mono repo

[philip.pfaffe]

Summary

When building polly as part of the monorepo (actually, as part of any setup
using LLVM_ENABLE_PROJECTS), the LLVMPolly library used in the lit tests ends
up in a different directory in the build tree than in an in-tree build

polly/trunk/test/CMakeLists.txt

 endif()
 if (POLLY_GTEST_AVAIL)
   list(APPEND POLLY_TEST_DEPS PollyUnitTests)
 endif ()
 
 set(LLVM_BINARY_DIR "${LLVM_BINARY_DIR}")
 set(LLVM_TOOLS_DIR "${LLVM_TOOLS_BINARY_DIR}")
 set(LLVM_LIBS_DIR "${LLVM_BINARY_DIR}/lib${LLVM_LIBDIR_SUFFIX}")
+if (CMAKE_LIBRARY_OUTPUT_DIRECTORY)
+  set(POLLY_LIB_DIR ${CMAKE_LIBRARY_OUTPUT_DIRECTORY})
+else()
   set(POLLY_LIB_DIR "${POLLY_BINARY_DIR}/lib")
+endif()
 
 configure_lit_site_cfg(
   ${CMAKE_CURRENT_SOURCE_DIR}/lit.site.cfg.in
   ${CMAKE_CURRENT_BINARY_DIR}/lit.site.cfg)
 
 # Run regression and unit tests
 add_lit_testsuite(check-polly-tests "Running polly regression tests"
   ${CMAKE_CURRENT_BINARY_DIR}