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On macOS with brew install llvm and llvm environment set up, 3.9 has stopped reporting OpenMP found with OpenMP_FOUND even on a CXX only project (but it still finds OpenMP and correctly sets the new OpenMP_CXX_FOUND flag).

Example CMakeLists.txt:

cmake_minimum_required(VERSION 3.8)

project(EXAMPLE LANGUAGES CXX)

find_package(OpenMP)

message(STATUS "Overall: ${OpenMP_FOUND}")
message(STATUS "CXX: ${OpenMP_CXX_FOUND}")

Output:

-- The CXX compiler identification is Clang 4.0.1
-- Check for working CXX compiler: /usr/local/opt/llvm/bin/clang++
-- Check for working CXX compiler: /usr/local/opt/llvm/bin/clang++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Found OpenMP_CXX: -fopenmp=libomp (found version "3.1")
-- Overall:
-- CXX: TRUE
-- Configuring done
-- Generating done
-- Build files have been written to: /Users/user/tmp

Exactly the same, just with CMake 3.8.2:

-- The CXX compiler identification is Clang 4.0.1
-- Check for working CXX compiler: /usr/local/opt/llvm/bin/clang++
-- Check for working CXX compiler: /usr/local/opt/llvm/bin/clang++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Try OpenMP CXX flag = [-fopenmp=libomp]
-- Performing Test OpenMP_FLAG_DETECTED
-- Performing Test OpenMP_FLAG_DETECTED - Success
-- Found OpenMP: -fopenmp=libomp
-- Overall: TRUE
-- CXX:
-- Configuring done
-- Generating done
-- Build files have been written to: /Users/user/tmp