Define libiGeom as a dependency of libmcnp2cad if part of a larger pr…

[pshriwise]

This fixes a bug I introduced in #43.

The if block here doesn't set IGEOM_LIB at all if mcnp2cad is part of a bigger project, so the iGeom library won't be linked (unless you were testing the previous change on in a directory with cached CMake variables like a fool...).

This PR addresses that problem by adding iGeom as a dependency of the mcnp2cad library by default. If the project is being configured as a standalone, an external iGeom library must be found.

[gonuke]

This change doesn't make sense to me...

[gonuke]

closed by accident!

[gonuke]

Doesn't this just hardcode something we IF'ed out of the flow?

[gonuke]

Don't we just need to make this a hard dependency with FIND_PACKAGE and fix the way we use it from the plugin, etc?

[pshriwise]

The problem with using only FIND_PACKAGE is that it will require en existing iGeom library during configuration of the plugin, which, given that the build command hasn'e been run yet, won't exist. This will cause the configuration to fail and lead us back to a situation where we require several make/cmake commands to successfully build the project.

My thought here is that we now cover the standalone case by using FIND_PACKAGE which would require that an iGeom library already exists somewhere on the system. If it is part of a larger project, that project is required to build an iGeom interface to link to, which is why I decided to hardcode this "library" as iGeom but really in this case it is target dependency that should exist as part of the plugin when the mcnp2cad library is being linked.

There may be better ways to do this, however. I know CMake has an ExternalProject module that can handle this as well, but I kind of liked the way we were doing it.

[gonuke]

Perhaps it would be better if we used some other trigger. For example, is it possible to have IGEOM_LIB set (or unset) before we enter this file. If it is already set, we can skip the FIND_PACKAGE? Or something similar?

[pshriwise]

I like that better, yeah. Leaves more flexibility for the library name. Updating..

I'll submit another PR to dagmc-trelis setting the IGEOM_LIB variable before configuring the mcnp2cad subdirectory.

[gonuke]

This definitely seems better - makes me wonder, now, whether we should treat IGEOM_INCLUDE_DIR within this, as well?

[pshriwise]

Good point. I'm not sure about that... are there clearly defined rules on how those will be found for an externally built iGeom library? Should we be looking at CGM to understand that?

[gonuke]

I'm happy with this for now, we can improve other parts more in the future.
Thanks @pshriwise