* Use gnuconfig package for config file replacement
Currently the autotools build system tries to pick up config.sub and
config.guess files from the system (in /usr/share) on arm and power.
This is introduces an implicit system dependency which we can avoid by
distributing config.guess and config.sub files in a separate package,
such as the new `gnuconfig` package which is very lightweight/text only
(unlike automake where we previously pulled these files from as a
backup). This PR adds `gnuconfig` as an unconditional build dependency
for arm and power archs.
In case the user needs a system version of config.sub and config.guess,
they are free to mark `gnuconfig` as an external package with the prefix
pointing to the directory containing the config files:
```yaml
gnuconfig:
externals:
- spec: gnuconfig@master
prefix: /tmp/tmp.ooBlkyAKdw/lol
buildable: false
```
Apart from that, this PR gives some better instructions for users when
replacing config files goes wrong.
* Mock needs this package too now, because autotools adds a depends_on
* Add documentation
* Make patch_config_files a prop, fix the docs, add integrations tests
* Make macOS happy
87450f3688
