use pkg-config to find msolve, otherwise try to run it

[dimpase]

spkg-config.m4 for msolve

This will fix #38315

So far, it's not possible to find msolve's version by running it, but it should be fixed in the future by algebraic-solving/msolve#147

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• I have created tests covering the changes.
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[dimpase]

there are no dependencies checks in spkg-configure.m4, as we only use an executable which is already self-sufficient.
(it would have to change if we had an library interface to msolve)

[mezzarobba]

Thank you! Lgtm.