pass cxx_flags to gloo cmake (#30857)

cmake/external/gloo.cmake

@@ -42,7 +42,7 @@ ExternalProject_Add(
     UPDATE_COMMAND        ""
     CONFIGURE_COMMAND     ""
     BUILD_COMMAND         mkdir -p ${GLOO_SOURCE_DIR}/build
-        && cd ${GLOO_SOURCE_DIR}/build && cmake .. && make
+        && cd ${GLOO_SOURCE_DIR}/build && cmake .. -DCMAKE_CXX_FLAGS=${CMAKE_CXX_FLAGS} && make
         && mkdir -p ${GLOO_LIBRARY_DIR} ${GLOO_INCLUDE_DIR}/gloo
     INSTALL_COMMAND      ${CMAKE_COMMAND} -E copy ${GLOO_SOURCE_DIR}/build/gloo/libgloo.a ${GLOO_LIBRARY_DIR}
     COMMAND              ${CMAKE_COMMAND} -E copy_directory "${GLOO_SOURCE_DIR}/gloo/" "${GLOO_INCLUDE_DIR}/gloo"