[Mingw-cross-env-list] Improve dependency metadata specification

[Tony Theodore]

Hi All,

I'd like to make some improvements to the way we handle dependencies
for the core toolchain and native libraries/tools we build.

At the moment, we use target dependencies so all native builds occur
before any cross-targets. This causes various issues such as `gcc`
requiring `cmake` or `binutils` requiring `pkgconf`.

Another problem is that we don't have a way of determining if a package
has an empty build rule because it's explicitly disabled or if it's a
source-only package.

I've raised a PR[1] for this which introduces:
  - explicit cross-target deps (i.e. `gcc` depends on $(BUILD)~gmp)
  - optional order-only deps which won't trigger rebuilds
  - virtual toolchain package `cc` which gathers `gcc`, `pkgconf`, and
    any general native tools
  - autotools/cmake are automatic order-only deps for those packages 
    that invoke them
  - better handling of disabled, source-only, and meta pkgs in both
    build status output, build-matrix[2], and input for `build-pkg`

The only issue I can see is if someone is relying on implied
dependencies that would currently be built with `gcc` alone (a message 
will be printed in this case). Building any other packages will cause 
any required packages to be built as normal.

`make gcc` is probably the longest-standing convention - we'll never
need to change it again so I'm hoping these changes won't cause any
issues or undue confusion.

Cheers,

Tony


[1] https://github.com/mxe/mxe/pull/1990 
[2] https://deploy-preview-1990--mxe.netlify.com/build-matrix.html