Install headers to spz prefix

[hobu]

This fixes #43 by installing the SPZ headers into ${CMAKE_INSTALL_INCLUDEDIR}/spz

[ProjitB]

I guess a slight concern here is that others using the build system may need to change their paths?

[hobu]

yes, this could result in software having to change its include paths or explicitly adding ${CMAKE_INSTALL_INCLUDEDIR}/spz as an include path when people are compiling existing code.

[ProjitB]

I still like the idea of doing this. Just thinking whether bumping our minor version would sufficient if we take in this change.

[hobu]

Just thinking whether bumping our minor version would sufficient if we take in this change.

IMO this change should happen in a major version release.

Speaking of which, I think it would be a good idea to add both library version and format version information.

[ProjitB]

Yeah we're still figuring out how to cleanly separate those (or at least the messaging of it). For now I'm going to add a label to this to target it for a v3.0.0 library release.