[bug#47972] Add irods.

[Ricardo Wurmus]

Hi Guix,

this set of patches adds irods and irods-client-icommands.  What 
makes this patch set a little annoying is the fact that irods 
insists on being built with clang 6; this means that we need 
variants of existing GCC-built packages just so that they can be 
linked with the irods libraries.

For the most part this is not too bad because we need specific 
versions of certain libraries anyway.  But for snappy-with-clang6 
and avro-cpp it’s pretty bad.  You’ll see that I’m using an inline 
package definition for a variant of avro-cpp that is built with 
clang; that package variant needs snappy, so I had to introduce 
snappy-with-clang6.

Before we can actually merge this we would have to move this 
avro-cpp variant to its own top-level package definition, because 
it turns out that irods-client-icommands also needs it.

If you have an idea how to build irods with GCC, I’d be very 
happy.  (It takes quite some time to build irods, so be prepared 
for long iteration cycles…)

--
Ricardo