Switch to the OpenMP threadpool (#1638)

* Use the Eigen OpenMP thread pool device

* CMake: Work around ancient non-compliant compilers used by the CI system

* Work around the broken CI system by making OpenMP optional

src/ngraph/runtime/cpu/CMakeLists.txt

@@ -14,6 +14,8 @@
 # limitations under the License.
 # ******************************************************************************
 
+include(FindOpenMP)
+
 set(SRC
     cpu_backend.cpp
     cpu_builder.cpp
@@ -154,6 +156,13 @@ if (NGRAPH_CPU_ENABLE)
         target_compile_definitions(cpu_backend PRIVATE "NGRAPH_DEX_ONLY")
     endif()
 
+    if(OPENMP_FOUND)
+        target_compile_options(cpu_backend PRIVATE "${OpenMP_CXX_FLAGS}")
+        target_compile_definitions(cpu_backend PRIVATE EIGEN_OPENMP)
+    else()
+         message(WARNING "The build toolset doesn't support OpenMP. This will impact performance and lead to slowdowns.")
+    endif()
+
     if(NGRAPH_DISTRIBUTED_ENABLE)
         find_package(MPI REQUIRED)
         add_definitions(-DNGRAPH_DISTRIBUTED)