Use dynamic libstdc++ selection only on Linux

[sloede]

In #853, we introduced the ability to bundle the libstdc++ library that is actually loaded by Julia with a library/app bundle. Before, it was hardcoded to use always the Julia-provided libstdc++ library, which did not work on Linux platforms that had a local system-provided libstdc++ newer than the Julia-provided one.

However, the solution implemented in #853 caused PackageCompiler.jl to not work anymore for Windows users without administrator privileges. This PR remedies this by making the dynamic libstdc++ selection only run on Linux, just like it is done in Julia itself.

Resolves #866.

[sloede]

@bgeihe Could you please confirm that this PR still works on your system with the newer libstdc++?

[codecov[bot]]

Codecov Report

Merging #869 (18ce162) into master (846292c) will decrease coverage by 0.68%. The diff coverage is 86.66%.

@@            Coverage Diff             @@
##           master     #869      +/-   ##
==========================================
- Coverage   84.94%   84.26%   -0.68%     
==========================================
  Files           3        3              
  Lines         797      801       +4     
==========================================
- Hits          677      675       -2     
- Misses        120      126       +6     

Files	Coverage Δ
src/PackageCompiler.jl	92.57% <86.66%> (-0.87%)	:arrow_down:

[benegee]

Confirmed!

[sloede]

All CI issues seem to be unrelated. I am merging this even though we do not have a single test passing on Julia v1.9 (again, unrelated failures it seems), since it is a major issue for many Windows users.