[CMake] INCLUDE_DIRECTORIES empty?

[David Doria]

If I do something like this:

INCLUDE_DIRECTORIES(${INCLUDE_DIRECTORIES}  ../../include)

I'd expect ../../include to be appended to the current INCLUDE_DIRECTORIES.

So then when I do this:
MESSAGE("INCLUDE_DIRECTORIES: ${INCLUDE_DIRECTORIES}")

at the very least, I should see ../../include, but probably other
paths before it. Instead, I see only:
INCLUDE_DIRECTORIES:

I'm pretty sure I have the syntax correct because if I do:

MESSAGE("CMAKE_BINARY_DIR: ${CMAKE_BINARY_DIR}")

I see:
CMAKE_BINARY_DIR: /media/portable/src/meshtoolboxVTK/src/SpinRecognize/bin

Any thoughts on why I can't see the contents of the variable
INCLUDE_DIRECTORIES?

Thanks,

David