Warn about potentially incompatible compiler

[sloede]

Does not really provide a solution to #738, but at least helps users understand what they can do about it.

[codecov]

Codecov Report

❌ Patch coverage is 73.68421% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.02%. Comparing base (466aec3) to head (5c7d27c).
⚠️ Report is 84 commits behind head on master.

Files with missing lines	Patch %	Lines
src/PackageCompiler.jl	73.68%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #886      +/-   ##
==========================================
- Coverage   84.26%   84.02%   -0.25%     
==========================================
  Files           3        3              
  Lines         801      820      +19     
==========================================
+ Hits          675      689      +14     
- Misses        126      131       +5     

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