Annotate pools with valgrind macros so that it detects errors in pool
usage. Typically, we wish valgrind to detect a leak when the user fails
to call proper deallocation function.

* spot/misc/fixpool.hh, spot/misc/mspool.hh: here
* configure.ac: ensure that valgrind header exists
* tests/Makefile.am, tests/core/mempool.cc, tests/core/mempool.test,
  tests/core/.gitignore: add tests to ensure valgrind accurately detects
  leaks