test: build with C++20 standard

[c-dilks]

Briefly, what does this PR introduce?

test for #115

[c-dilks]

same issue....

[wdconinc]

It seems you build and install EICrecon into prefix/ in build_2, then try to build EICrecon again in build_3.

#pragma once doesn't like that you have -I the prefix, and -I the same directory in the second EICrecon source tree. That's (arguably) the main drawback of #pragma once, but in this case it's probably just helping to surface an issue that you want to address: you probably don't intend to build EICrecon twice.

[c-dilks]

Ah, good catch. I wonder why this hasn't surfaced before. We likely no longer need to test re-building parts of richgeo to a separate shared library.