[lmi] PATCH: Fix CI after C++20 and fs::path changes

[Vadim Zeitlin]

 Hello,

 My branch with the changes, which should be accessible as xanadu/ci-cxx20
after git-fetch for you (for the reference, this branch corresponds to
https://github.com/let-me-illustrate/lmi/pull/185), is finally ready for
merging as all the CI builds pass.

 There are many commits there, that I'm not going to describe to avoid
scaring you, especially because only just a few of them change files other
than .github/workflows/ci.yml, Makefile.am and configure.ac, that you
shouldn't normally worry about.

 The remaining change undoes my own commit to boost_regex.hpp, which should
hopefully be allowed too (as the classic said, “I brought you into this
world. It is on me to take it away.”, and classic are always right), a tiny
clang-specific change to expression_template_0_test.cpp and less trivial
but still simple changes to path_utility_test.cpp and
expression_template_0_test.cpp that were discussed in more details in this
post: https://lists.nongnu.org/archive/html/lmi/2021-05/msg00027.html

 As I had previously said, I feel almost physically uncomfortable with the
changes being pushed while the CI builds are failing, so it would be great
if this could be merged soon. I'll definitely wait until it is before
submitting any other changes, so that they could be tested by the CI, at
any rate.

 Thanks in advance!
VZ

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