cmake HAVE_IBV_EXP examples

ceph/ceph cmake/modules/Findverbs.cmake :17 

CHECK_CXX_SOURCE_COMPILES("
    #include <infiniband/verbs.h>
    int main() {
      struct ibv_context* ctxt;
      struct ibv_exp_gid_attr gid_attr;
      ibv_exp_query_gid_attr(ctxt, 1, 0, &gid_attr);
      return 0;
    } " HAVE_IBV_EXP)