Hello,

I configured mpich/main (or 4.0a2) with 
   ./configure  CC=gcc CXX=g++ --enable-fortran=no
on Mac with Apple clang version 12.0.5 (clang-1205.0.22.11). The build failed with

  ...
  CCLD     hydra_pmi_proxy
Undefined symbols for architecture x86_64:
  "_hwloc_topology_set_io_types_filter", referenced from:
      _HYDT_topo_hwloc_init in libhydra.a(topo_hwloc.o)

--Junchao Zhang