Handle signatures for source and library separately

[Martin Sandve Alnæs]

A more optimal handling of signatures would be like this:

• user provides src_signature, a code generator, and build options.
• dijitso sets lib_signature to a hash of src_signature and build options
• if lib_signature in cache -> don't need to build
• if src_signature in cachedir/src/ -> don't need to call code generator (but still need to build)

[Martin Sandve Alnæs]

Not doing this. It's not a big advantage and adds complexity to the cache mechanism that we'd rather keep simple.