How to build dependencies separately

[adamjstewart]

I'm trying to add a package for QNNPACK to the Spack package manager. I see that QNNPACK downloads its own dependencies, and that this can be avoided by setting *_SOURCE_DIR via cmake. Is there a way to point to an existing external installation instead of a source directory so that Spack doesn't need to rebuild all of these dependencies? Spack is designed to work on air-gapped supercomputers that don't have internet access, so I can't have it download anything at build time.