Before, we'd always use `cc = null`, and check for that. The problem is
this breaks for cross compilation to platforms that don't support a C
compiler.

It's a very subtle issue. One might think there is no problem because we
have `stdenvNoCC`, and presumably one would only build derivations that
use that. The problem is that one still wants to use tools at build-time
that are themselves built with a C compiler, and those are gotten via
"splicing". The runtime version of those deps will explode, but the
build time / `buildPackages` versions of those deps will be fine, and
splicing attempts to work this by using `builtins.tryEval` to filter out
any broken "higher priority" packages (runtime is the default and
highest priority) so that both `foo` and `foo.nativeDrv` works.

However, `tryEval` only catches certain evaluation failures (e.g.
exceptions), and not arbitrary failures (such as `cc.attr` when `cc` is
null). This means `tryEval` fails to let us use our build time deps, and
everything comes apart.

The right solution is, as usually, to get rid of splicing. Or, baring
that, to make it so `foo` never works and one has to explicitly do
`foo.*`. But that is a much larger change, and certaily one unsuitable
to be backported to stable.

Given that, we instead make an exception-throwing `cc` attribute, and
create a `hasCC` attribute for those derivations which wish to
condtionally use a C compiler: instead of doing `stdenv.cc or null ==
null` or something similar, one does `stdenv.hasCC`. This allows quering
without "tripping" the exception, while also allowing `tryEval` to work.

No platform without a C compiler is yet wired up by default. That will
be done in a following commit.

This platform doesn't have a C compiler, and so relies and the changes
in the previous commit to work.

This is GHCJS, and perhaps other obscure targets.

…ut-cc

js-ghcjs didn't fit in an existing categor.

js-ghcjs didn't fit in an existing categor.

…into ghcjs-cross-without-cc

Use `buildPackages.stdenv.mkDerivation` because we are making a shell
script to start hoogle on the build platform.

This is a bit dubvious, but the alternative of making nodejs a
nativeBuildInput for node packages is worse. In general the cross story
for interpreted languages is murky, and this fits that pattern.

This makes us a bit more robust to various splicing nastiness. May splicing
someday go so we don't have to resort to such hacks.

See c3c245d

(cherry picked from commit 59dbb00)

Otherwise it passes `--with-ghc=ghc`, and we do the wrong thing.

… into ghcjs-cross-without-cc