crossOverlays only apply to the packages being built, not the build
packages. It is useful when you don’t care what is used to build your
packages, just what is being built. The idea relies heavily on the
cross compiling infrastructure. Using this implies that we need to
create a cross stdenv.

(cherry picked from commit a3a6ad7)

- makeStaticBinaries don’t work on Darwin (no stable ABI!)
- Need to make sure NIX_CFLAGS_LINK appends
- isStatic is not used anymore

(cherry picked from commit 8726f6a)

The conditional was incorrect - postInstall script should only hsppen
when enableShared = true.

(cherry picked from commit 5e06294)

This is kind of a mess, but basically:

- static=true, shared=true means to build statically but move it to
  the static output
- static=true, shared=false means to build statically and leave it in
  the main output
- static=false, shared=true means to not build static at all

Confusingly, the old default was static=true, shared=true even though
static=false? Still can’t figure out what was meant by that.

(cherry picked from commit e999def)

when enableShared = false, we set --without-shared flag.

(cherry picked from commit 53a6789)

Adds the static overlay that can be used to build Nixpkgs statically.
Can be used like:

  nix build pkgsStatic.hello

Not all packages build, as some rely on dynamic linking.

(cherry picked from commit 6d90a8b)