Add configure option --enable-helgrind

Add configure option --enable-helgrind

Like --enable-valgrind but uses helgrind instead of memcheck.

If both --enable-valgrind and --enable-helgrind are given then
helgrind takes priority.

--enable-helgrind requires --enable-valgrind-annotations.

	* configure.ac: Add --enable-helgrind option.
	* tests/Makefile.am: If USE_HELGRIND is true, then include
	--tool=helgrind in the valgrind command that tests are run
	under.

Signed-off-by: Aaron Merey <amerey@redhat.com>