[Chapel Merge] Improve CHPL_LIBFABRIC handling.

Branch: refs/heads/1.23-for-EX
Revision: ec49b78
Author: gbtitus
Log Message:

Merge pull request #16686 from gbtitus/dup-e5584b1

Improve CHPL_LIBFABRIC handling.

(Duplicate PR #16605 from master to 1.23-for-EX.)

(Original PR reviewed by @ronawho.)

Make a couple of adjustments to the CHPL_LIBFABRIC handling. The main
change is to default to CHPL_LIBFABRIC=system whenever pkg-config exists
and can find a libfabric, on the assumption that a system one is more
likely to match up with the system’s characteristics than our bundled
one is. While here, also add CHPL_LIBFABRIC to the list of config vars
that can override our defaults, getting rid of an incorrect and annoying
but nevertheless harmless message during compilation.

Modified Files:
M util/chplenv/chpl_libfabric.py
M util/chplenv/overrides.py
M util/chplenv/utils.py

Compare: https://github.com/chapel-lang/chapel/compare/4706bf36b51a...ec49b78c2f1b