[DOC] add remark about how to set CC and CXX vars

[SGSSGene]

This adds a remark on how to set CC and CXX.

Also, added a comment about failing to find SDSL even though the problem lies with apple clang. see https://github.com/seqan/seqan3/issues/3046

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Name	Status	Preview	Updated
seqan3	✅ Ready (Inspect)	Visit Preview	Oct 24, 2022 at 8:08AM (UTC)

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Codecov Report

Base: 98.25% // Head: 98.25% // No change to project coverage :thumbsup:

Coverage data is based on head (e1db405) compared to base (dc497ac). Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3049   +/-   ##
=======================================
  Coverage   98.25%   98.25%           
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  Files         276      276           
  Lines       12344    12344           
=======================================
  Hits        12128    12128           
  Misses        216      216           

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[eseiler]

We explain how to use -DCMAKE_CXX_COMPILER further down. I would kinda prefer to set it in cmake, because it persists in a new shell session.

The sdsl error could be generalized for all submodules, sdsl is just currently the first one we check.

[eseiler]

@SGSSGene I pushed a suggestion, what do you think?

[SGSSGene]

Good suggestion!