erasure-code: isa plugin must link with ErasureCode.cc

Otherwise it will not get the methods it needs. A test is added to check
the plugin loads as expected, from the command line.

Signed-off-by: Loic Dachary <loic@dachary.org>
We can't/don't build the ISA plugin on ubuntu precise or rhel6, i believe, because we need yasm 1.2 and they only have 1.1.  This is a problem for putting this through QA too because not all machines in the pool will have ISA support.

We could install a backport yasm 1.2 on our gitbuilder machines for the benefit of QA.  That leaves out release packages, though, and I don't think the build-requires: etc will let us automagically pull down a newer yasm to do the build.

So... I suspect what we need is some conditional here to query whether ISA support is built in...