Investigate running tests under helgrind

[brson]

I don't know much about this tool, but it might be useful for exercising our task system.

[eholk]

I've tried using helgrind a little. My perception was that there are far too many false positives to be useful. I think even a trivial "Hello World" written in C will cause helgrind errors from the C runtime.

[brson]

I've been using helgrind quite often to debug task issues. There are some (likely) false positives, but they are consistent. With a suppressions file I think it could definitely be worthwhile to have an automated helgrind run of the testsuite.

[brson]

I've added an --enable-helgrind option to the configure script. My project for the weekend is to make the build helgrind-clean.

[brson]

The build is helgrind clean.