This allows various applications. It allows users to set global
optimisation flags, e.g.
stdenv.userHook = ''NIX_CFLAGS_COMPILE+=" -funroll-loops"'';
But the impetus is as an alternative to issue #229, allowing impure
stdenv setup for people who want to use distcc:
stdenv.userHook = "source /my/impure/setup-script.sh";
This is probably a bad idea, but at least now it's a bad idea in
people's configuration and not in Nixpkgs. :-)
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