Add a config option to strip `-Werror*` or `-Werror=*` from compile lines everywhere.
```yaml
config:
keep_werror: false
```
By default, we strip all `-Werror` arguments out of compile lines, to avoid unwanted
failures when upgrading compilers. You can re-enable `-Werror` in your builds if
you really want to, with either:
```yaml
config:
keep_werror: all
```
or to keep *just* specific `-Werror=XXX` args:
```yaml
config:
keep_werror: specific
```
This should make swapping in newer versions of compilers much smoother when
maintainers have decided to enable `-Werror` by default.
330832c22c