support pmix 3.2.3

[garlick]

Problem: TOSS4 apparently ships a pmix-3.2.3 package which would make packaging flux-pmix as an RPM a bit easier, but we require v >= 4.0.0.

rzvernal11 /g/g0/garlick/proj/flux-pmix > rpm -qi pmix
Name        : pmix
Epoch       : 1
Version     : 3.2.3
Release     : 1.4.t4
Architecture: x86_64
Install Date: Tue 28 Mar 2023 12:01:08 PM PDT
Group       : Development/Libraries
Size        : 6231662
License     : BSD
Signature   : RSA/SHA1, Mon 11 Apr 2022 06:34:16 PM PDT, Key ID 7d5b09f4c22a3daf
Source RPM  : pmix-3.2.3-1.4.t4.src.rpm
Build Date  : Mon 11 Apr 2022 06:31:16 PM PDT
Build Host  : builder2-x86.buildfarm.internal
Relocations : /usr 
Packager    : redhat
Vendor      : redhat
Summary     : An extended/exascale implementation of the PMIx Standard

This package installs to the normal system prefix e.g.

/usr/lib64/libpmix.la
/usr/lib64/libpmix.so
/usr/lib64/libpmix.so.2
/usr/lib64/libpmix.so.2.2.33

/usr/include/pmi.h
/usr/include/pmi2.h
/usr/include/pmix.h
/usr/include/pmix_common.h
/usr/include/pmix_extend.h
/usr/include/pmix_server.h
/usr/include/pmix_tool.h
/usr/include/pmix_version.h

[garlick]

Should have been closed by #80