- The env variable name should be Xenomai_ROOT_DIR (not all caps)

- I personally like the Package_ROOT_DIR variables to be a complete override of the default search paths, and not just an additional hint. This allows to pick a nonstandard installation even when another version exists in the default system path. I enforce this with the 
[NO_DEFAULT_PATH](https://github.com/ros/cmake_modules/blob/master/cmake/Modules/FindTinyXML.cmake#L52) option.