@jwaldrop107 Technically, the Spack build still does not work, even with this fix. However, it is an internal issue to CMaize that needs to be fixed, so I don't think it is necessary as part of this PR. Therefore, I'd say this PR is r2g.

Here is the explanation of the issue:

cmaize_find_or_build_optional_dependency() calls target_compile_definitions() on the target (L43) after it finds the target, which results in the following error when Eigen was not built as part of the sigma build:

-- Attempting to find installed eigen
Looking for eigen
Creating a Git dependency
Was already found? FALSE
-- eigen installation found
CMake Error at /tmp/zachcran/spack-stage/spack-stage-sigma-master-lntoxru2oivacwbpl3rwcj3jlqi2yjg3/spack-build-lntoxru/_deps/cmaize-src/cmake/cmaize/user_api/dependencies/find_or_build_optional_dependency.cmake:42 (target_compile_definitions):
  Cannot specify compile definitions for target "eigen" which is not built by
  this project.

A workaround can also be applied in sigma by using cmaize_find_or_build_dependency() and wrapping the call with an if("${ENABLE_EIGEN_SUPPORT}"), if you want a fix faster.

NOTE: I have duplicated this explanation to an issue (#25) to better track it.

@zachcran I would say we go ahead and merge this, working under the assumption that you'll have the CMaize fix in place shortly. If that fix starts to drag out, the workaround you proposed can be added in a separate PR.