[openmp] - Ensure that -fopenmp is used with OpenMP::omp target (llvm#4337)

Author: ronlieb

openmp/runtime/openmp-config.cmake.in

@@ -11,3 +11,7 @@ set_and_check( openmp_INCLUDE_DIR "@PACKAGE_LIBOMP_HEADERS_INSTALL_PATH@" )
 set_and_check( openmp_INCLUDE_DIRS "${openmp_INCLUDE_DIR}" )
 
 include( "${CMAKE_CURRENT_LIST_DIR}/openmpTargets.cmake" )
+
+set_property(TARGET OpenMP::omp APPEND PROPERTY
+    INTERFACE_COMPILE_OPTIONS "-fopenmp"
+)