Making without PTPUSB still installs headers

Because header installation is a wildcard copy, the PTPUSB header is still installed, even if PTPUSB support isn't built.  One option would be to do an rm PTPUSB.hpp if HAS_USB = false, but this seems sloppy.

Determine a better system to copy headers, and not include PTPUSB.hpp if HAS_USB is false
