Fix OpenMP build with libomp 15+ (#410)

CMakeLists.txt

@@ -1,6 +1,6 @@
 set (CMAKE_FIND_NO_INSTALL_PREFIX TRUE FORCE)
 cmake_minimum_required (VERSION 3.13)
-project(treelite LANGUAGES CXX C VERSION 3.0.0)
+project(treelite LANGUAGES CXX C VERSION 3.1.0)
 
 set(CMAKE_CXX_STANDARD 14)
 

src/CMakeLists.txt

@@ -12,12 +12,28 @@ endif(UNIX)
 add_library(objtreelite_common OBJECT)  # Component shared by both main package and runtime
 
 if(USE_OPENMP)
-  if (APPLE)
+  if(APPLE)
     # Require CMake 3.16+ on Mac OSX, as previous versions of CMake had trouble locating
     # OpenMP on Mac. See https://github.com/dmlc/xgboost/pull/5146#issuecomment-568312706
     cmake_minimum_required(VERSION 3.16)
-  endif (APPLE)
-  find_package(OpenMP REQUIRED)
+    find_package(OpenMP)
+    if (NOT OpenMP_FOUND)
+      # Try again with extra path info; required for libomp 15+ from Homebrew
+      execute_process(COMMAND brew --prefix libomp
+          OUTPUT_VARIABLE HOMEBREW_LIBOMP_PREFIX
+          OUTPUT_STRIP_TRAILING_WHITESPACE)
+      set(OpenMP_C_FLAGS
+          "-Xpreprocessor -fopenmp -I${HOMEBREW_LIBOMP_PREFIX}/include")
+      set(OpenMP_CXX_FLAGS
+          "-Xpreprocessor -fopenmp -I${HOMEBREW_LIBOMP_PREFIX}/include")
+      set(OpenMP_C_LIB_NAMES omp)
+      set(OpenMP_CXX_LIB_NAMES omp)
+      set(OpenMP_omp_LIBRARY ${HOMEBREW_LIBOMP_PREFIX}/lib/libomp.dylib)
+      find_package(OpenMP REQUIRED)
+    endif()
+  else()
+    find_package(OpenMP REQUIRED)
+  endif()
 else()
   message(STATUS "Disabling OpenMP")
 endif()