add warning about threading issue

[IanButterworth]

Fixes #778

IIUC in non-incremental (--sysimage=*.ji) mode with stdlibs filtered out, julia has to do a lot more bootstrapping, which may max out all available threads, so not maxing out all threads during each Pkg.precompile subprocess isn't guaranteed, so it's best to warn the user to just be conservative with enabling threading.

cc. @vtjnash

[codecov[bot]]

Codecov Report

Merging #781 (0301e62) into master (8e4c03c) will decrease coverage by 0.10%. The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master     #781      +/-   ##
==========================================
- Coverage   83.00%   82.90%   -0.10%     
==========================================
  Files           3        3              
  Lines         665      667       +2     
==========================================
+ Hits          552      553       +1     
- Misses        113      114       +1     

Impacted Files	Coverage Δ
src/PackageCompiler.jl	92.95% <50.00%> (-0.17%)	:arrow_down:

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[JeffBezanson]

We should just fix this for real instead!

[sjkelly]

I am pretty sure this is fixed!