Customizing install_manifest.txt location (or raising an error if not found)

[ihnorton]

Re: #996

I think it would be useful to:

• raise an error (or at least a warning) if no install_manifest.txt is found in the CMAKE_BUILD_DIR, which can happen with eg a superbuild install. Considering this the key link for Python packaging to pick up the skbuild/CMake artifacts, a missing install_manifest is likely a problem in most circumstances.
• possibly allow customization of where to look for the manifest.

If either/both of these are of interest, I can make a PR.

[henryiii]

I think we could recursively collect install_manifests?