[lmi] PATCH: define LMI_GCC and LMI_CLANG

[Vadim Zeitlin]

 Hello,

 Before I forget about it completely, I'd like to resubmit my changes
adding LMI_GCC symbol, which were previously discussed and even included in
another PR, but not merged yet.

 The originally discussed change is just this commit:

https://github.com/let-me-illustrate/lmi/pull/176/commits/c49d62e16a992b0de221b9e75913440fb4d9cd95

and while it could still be merged/cherry-picked on its own, just to get
rid of the repeated "defined __GNUC__ && !defined __clang__" tests (which
was the original motivation), I think it would be better to merge all the
changes from

        https://github.com/let-me-illustrate/lmi/pull/176/

instead.

 The two next commits it contains are not really related to LMI_XXX symbols
at all, but just do some minor and harmless cleanup, please see their
descriptions at

https://github.com/let-me-illustrate/lmi/pull/176/commits/b96d25594aacb229be3d4d07504e5659678ba64d

and

https://github.com/let-me-illustrate/lmi/pull/176/commits/b726be7f44743c3031e4cbafff4d904d94d6d56a

respectively.

 The final commit, at

https://github.com/let-me-illustrate/lmi/pull/176/commits/ca970f9aa3bc075e5f2bc0ef20a467ed6ac52641

is a bit more extensive and hence potentially more controversial, but I
think it does make sense to define LMI_CLANG too, as we already define
LMI_MSC and the first commit above adds LMI_GCC definition. However this
one can be omitted if you don't like it for any reason.


 The entire combined diff can, as always, be seen at

        https://github.com/let-me-illustrate/lmi/pull/176/files

and doesn't touch any lines of code at all, just the preprocessor tests.
Please let me know if you have any questions or comments about these
changes and, if not, thanks in advance for merging them!

VZ

P.S. You may notice that MSW CI build has failed for this PR. This is
     entirely unrelated to the changes in it, however, and is just due
     to the fact that there seems to be a problem with xvfb/wine in the
     CI environment. I'm aware of it and will try to fix it, but it's
     time-consuming because I can't reproduce it locally, so it might
     take a bit more time.

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