Coming back to this, is Player expecting an environment variable called XSENSMT_SOURCE_DIR? I tried setting this and it does not seem to work, neither could I find a Cmake variable called XSENS_SOURCE_DIR.
I have the source code required, I think (/CmtSrc from the Xsens CD).
Due to licensing issues, the source code used by the XSensMT driver to talk to the device can't be included in Player. If you want to use that driver, you need to provide the code yourself (it's supplied by the manufacturer) and set XSENSMT_SOURCE_DIR to point to where you've put it. If you don't want to use that driver, you can safely ignore the message.