Add opt-in auto precompilation after Pkg.add, instantiate, update

[IanButterworth]

Runs Pkg.precompile() after Pkg.add(), instantiate(), update()

Enabled by setting ENV["JULIA_PKG_PRECOMPILE_AUTO"]=1 etc.

[staticfloat]

It's going to be a string; you want something like if parse(Int, get(ENV, "JULIA_PKG_PRECOMPILE_AUTO", "0")) == 1

[IanButterworth]

What would be best? I didn't mean to pick a strategy.

Edit: I reviewed https://docs.julialang.org/en/v1/manual/environment-variables/ and that approach seems typical

[tkf]

Setting environment variables is not the cleanest way to configure the behavior of a package JuliaLang/julia#34726. Why not use (run-time) Preferences.jl?

[KristofferC]

This is just a way for people to be able to try it out to collect feedback. It is not a permanent setting.