Revert "Removing extra guard against multiple inclusion (we are already using #pragma once."

This reverts commit ac96db504120bc47eacdfeea6ed84e60cfc3d7d5.

Conflicts:
	vrpn_OneEuroFilter.h
Revert "Removed defines to avoid multiple header inclusion when we already used #pragma once."

This reverts commit 3941ff286de283be6e6423bc125bb957c2d98823.
Use just plain ifdef guards instead of #pragma once, more standard.
Add missing include
Fixes from clang-check -fixit
Improve comments in cmake
Don't force global properties if we're a subproject.
More CMake cleanup
Don't install HIDAPI header
Re-arrange the cmake file a little