[CMake] Appending paths to a variable

[Robert Bielik]

> To retrieve a variable from a subdirectory (added via
> ADD_SUBDIRECTORY), you have to do something like:
> GET_DIRECTORY_PROPERTY (MYVAR DIRECTORY subdirectory DEFINITION INCL_DIRS)
> Have a look at cmake documentation for more info about GET_DIRECTORY_PROPERTY.

Thanks Olivier, that was what I was looking for. However, I found that it was easier to accomplish
what I wanted with INCLUDE(...) instead.

/R