RFC: Retire meson.build files due to bit-rot

[BsAtHome]

The meson.build files were introduced in #619 afaics. They have been allowed to accumulate missing bits, bytes, characters and more over the past seven years. You cannot build LinuxCNC using a meson build. No documentation is covered and no HAL is covered. IMO, it is redundant and should be removed.

Any comments?

[andypugh]

Are the associated unit tests still present? I am not clear if one was dependent on the other.

[BsAtHome]

The unit tests are in [gitroot]/unit_tests/ and yes, they depend on meson. However, from the looks of it, all the tests have probably (quite) some bit-rot too.
The whole meson stuff will not build because we have been reorganising a lot (like the includes). Most tests are about the interpreter. Currently, the interpreter cannot be build out-of-tree because it hooks into a lot of core stuff (I remember disabling the test until it can be looked at and separated out in a proper way). The other test is about blendmath, but that seems not to be doing much interesting. I don't think anyone has run any of the unit tests since they were added 7 years ago.

One change happened 2 years ago, but that was fixing a typo in a comment...

[grandixximo]

I support the removal. The only consumer of the meson build is unit_tests/, so I audited it and salvaged the worthwhile content as regular runtests in #4281 as a first cleanup step: the blendmath test, plus a new G20/G21 unit-conversion runtest which was the only genuine coverage gap.

The rest of unit_tests/interp duplicates existing runtests under tests/interp/ (the unit tests were originally derived from them) or enshrines behavior the original author flagged as wrong with FIXME comments, so nothing of value is lost when the meson files go.