[CI] Add a -Wall -Werror build

[marcalff]

Add a build in CI to detect compilation warnings, and fail the build on warnings.

To test with a recent version of GCC and/or CLANG.

The build in general should be clean.

In particular, user visible headers (like the API) should be warning free,
to allow the user code to build warning free.

Adding a CI build to enforce this will help to maintain clean code.

[sirzooro]

Please build using both gcc and clang if possible. Both compilers have different diagnostics, so some issues are detected by one of them only.

[marcalff]

Please build using both gcc and clang if possible. Both compilers have different diagnostics, so some issues are detected by one of them only.

Yes, planning both.