Using `-Werror` is good practice for development and testing, but causes us a great
deal of heartburn supporting multiple compiler versions, especially as newer compiler
versions add warnings for released packages. This PR adds support for suppressing
`-Werror` through spack's compiler wrappers. There are currently three modes for
the `flags:keep_werror` setting:
* `none`: (default) cancel all `-Werror`, `-Werror=*` and `-Werror-*` flags by
converting them to `-Wno-error[=]*` flags
* `specific`: preserve explicitly selected warnings as errors, such as
`-Werror=format-truncation`, but reverse the blanket `-Werror`
* `all`: keeps all `-Werror` flags
These can be set globally in config.yaml, through the config command-line flags, or
overridden by a particular package (some packages use Werror as a proxy for determining
support for other compiler features). We chose to use this approach because:
1. removing `-Werror` flags entirely broke *many* build systems, especially autoconf
based ones, because of things like checking `-Werror=feature` and making the
assumption that if that did not error other flags related to that feature would also work
2. Attempting to preserve `-Werror` in some phases but not others caused similar issues
3. The per-package setting came about because some packages, even with all these
protections, still use `-Werror` unsafely. Currently there are roughly 3 such packages
known.
0182603609