In `configure.ac` the `AX_COMPILER_FLAGS` macro uses the result of
`AX_IS_RELEASE` to detect release builds and turn off `-Werror` in that case.

To be on a release build you need to fall in one of the two cases below:
* there's a `.tarball-version` in the project root with the desired source
  version in it
* `git describe --abbrev=4 '--match=v*' HEAD` gives you a name without any
  dash `-` in it (that is, you're exactly on the comming pointed to by an
  annotated tag)

Release orig tarballs should all carry `.tarball-version` so no further action
is needed.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>