Add ability to turn of Conduit dependency in Spack package

[white238]

Someone might want a version of Axom without Conduit/Sidre but with the rest of our functionality.

[rhornung67]

Don't we have that situation with some of our codes already?

[gzagaris]

Don't we have that situation with some of our codes already?

To the best of my knowledge, we're all building with hdf5 and conduit which are the main external dependencies for Sidre.

I think it would be nice to have the ability to build without these dependencies easily through spack.

[rhornung67]

I agree. I just thought people were doing this already.

[kennyweiss]

Seems like a reasonable idea.

@rhornung67 -- We were building that way in travis for our builds that did not have TPLs, since sidre is the only component that requires TPLs (the rest have optional dependencies). But, those builds were not run through spack.

I guess the benefit of being able to turn off sidre would be to generate an appropriate host-config for a given SYS_TYPE, and to be able to build other optional dependencies.

[rhornung67]

Possibilities:

• Make Conduit optional dependency and turn off all components that use Conduit if it's off.
• others?

Also add Spack configuration to test this.