Add compiler_flags.jl to avoid running julia-config.jl

[lucatrv]

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[codecov-io]

Codecov Report

Merging #94 into master will increase coverage by 2.22%.
The diff coverage is 82.92%.

@@            Coverage Diff             @@
##           master      #94      +/-   ##
==========================================
+ Coverage   70.09%   72.32%   +2.22%     
==========================================
  Files           5        6       +1     
  Lines         311      336      +25     
==========================================
+ Hits          218      243      +25     
  Misses         93       93

Impacted Files	Coverage Δ
src/PackageCompiler.jl	49.2% <ø> (ø)	⬆️
src/system_image.jl	82.35% <ø> (ø)	⬆️
src/static_julia.jl	74.43% <100%> (+3.1%)	⬆️
src/compiler_flags.jl	80% <80%> (ø)

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