This adds a "make check-valgrind" target which will run the test suite
under two Valgrind tools in succession: memcheck and helgrind. Requires
a bit of surgery on the shell script test runners, because
AX_VALGRIND_CHECK mainly assumes your tests are binaries.

The Valgrind tools drd and sgcheck are also available, but disabled by
default because they seem to crash on SpiderMonkey's threading.

https://bugzilla.gnome.org/show_bug.cgi?id=786995