Introduce --enable-valgrind-suppressions configure switch
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.
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