build: make the ompi prereq optional

[garlick]

Problem: the project cannot be built without openmpi, but it doesn't really depend on it, just a few tests.

Make openmpi optional and skip those tests if it's not configured.

Fix some outdated stuff in the configure.ac while we're at it.

[codecov[bot]]

Codecov Report

Merging #73 (27515e4) into main (8f8281d) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #73   +/-   ##
=======================================
  Coverage   79.44%   79.44%           
=======================================
  Files          10       10           
  Lines        1231     1231           
=======================================
  Hits          978      978           
  Misses        253      253           

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[garlick]

Thanks! Setting MWP.