The rationale for this is:

* Developers configure with `--enable-valgrind` and run `make check` to
  see all the error reports (`make check` continues testing even if
  there are Valgrind issues). This encourages us to fix these issues,
  especially if we are modifying those parts of the code as part of
  another bug.

* Builders configure with `--enable-valgrind
  --enable-valgrind-suppressions` and run `make check` with
  suppressions. `make check` doesn't show existing suppressed issues and
  stops building if it finds a new Valgrind issue that is not
  suppressed.