[lmi] PATCH: Build fixes after unwind.cpp addition

[Vadim Zeitlin]

 Hello,

 I've created https://github.com/let-me-illustrate/lmi/pull/172 with a
couple of small build fixes that are required to build lmi in the CI
environment, build it using autotools and/or build it using clang since
unwind.cpp addition.

 To describe the commits included in this PR in a bit more details:

- The first one is required to fix the CI build failures. I hadn't even
  noticed it, but https://github.com/let-me-illustrate/lmi/actions shows
  only failed builds since the addition of unwind.cpp due to the missing
  libraries and this simple change fixes it and should hopefully be quite
  uncontroversial as it just installs the required packages.

- The next commit is required to fix the autotools build even when using
  gcc and only touches configure.ac, so there shouldn't be any problems
  with it neither.

- The next, third commit, is required to fix the build when using clang and
  is the first one there might be some doubts about, because this change
  simply avoids compiling the new code when using clang. I don't know if
  it's worth trying to make it work with clang/libc++ instead, but even if
  you think that it is, I'd still prefer to do it later and for now just
  apply this commit to fix the clang build.

- The remaining two commits middle are not really required and could
  be omitted, but seemed like they could be useful, so I've included them
  in the same PR. Please feel free to apply or exclude them as you prefer.


 If possible, I'd really like to merge the first two and, preferably,
three (and, ideally, of course, all 5), commits soon. Please let me know if
you see any problems with doing this or any questions about the changes
here.

 Thanks in advance!
VZ

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